I was trying to clean cat hair off of my laptop, and swiped my hand gently across the touchpad. The X server crashed:

Xorg: ../src/evdev-mt-touchpad-tap.c:1030: tp_tap_handle_state: Assertion `tp->tap.nfingers_down >= 1' failed.

Thread 2 "InputThread" received signal SIGABRT, Aborted.
[Switching to Thread 0x7f58e7254700 (LWP 11644)]
0x00007f58ee846860 in raise () from /usr/lib/libc.so.6
(gdb) bt
#0  0x00007f58ee846860 in raise () at /usr/lib/libc.so.6
#1  0x00007f58ee847ec9 in abort () at /usr/lib/libc.so.6
#2  0x00007f58ee83f0bc in __assert_fail_base () at /usr/lib/libc.so.6
#3  0x00007f58ee83f133 in  () at /usr/lib/libc.so.6
#4  0x00007f58e7f0e2f0 in tp_tap_handle_state (tp=0x5619dd09aab0, time=2162337668) at ../src/evdev-mt-touchpad-tap.c:1030
#5  0x00007f58e7f08d17 in tp_post_events (tp=0x5619dd09aab0, time=2162337668) at ../src/evdev-mt-touchpad.c:1561
#6  0x00007f58e7f08e03 in tp_handle_state (tp=0x5619dd09aab0, time=2162337668) at ../src/evdev-mt-touchpad.c:1586
#7  0x00007f58e7f08ed6 in tp_interface_process (dispatch=0x5619dd09aab0, device=0x5619dd0976b0, e=0x7f58e7252dc0, time=2162337668) at ../src/evdev-mt-touchpad.c:1633
#8  0x00007f58e7efabe2 in evdev_process_event (device=0x5619dd0976b0, e=0x7f58e7252dc0) at ../src/evdev.c:857
#9  0x00007f58e7efac27 in evdev_device_dispatch_one (device=0x5619dd0976b0, ev=0x7f58e7252dc0) at ../src/evdev.c:865
#10 0x00007f58e7efae0c in evdev_device_dispatch (data=0x5619dd0976b0) at ../src/evdev.c:924
#11 0x00007f58e7ef4f3b in libinput_dispatch (libinput=0x5619dd021580) at ../src/libinput.c:1989
#12 0x00007f58e8141984 in xf86libinput_read_input (pInfo=0x5619dd040690) at xf86libinput.c:2180
#13 0x00005619dc6e8163 in  ()
#14 0x00005619dc6ea861 in  ()
#15 0x00005619dc6e7fae in  ()
#16 0x00007f58eebd008c in start_thread () at /usr/lib/libpthread.so.0
#17 0x00007f58ee907e7f in clone () at /usr/lib/libc.so.6

I'm using a debug build of libinput built from commit 5b29be3998d5f3670ec66147abc32e7d992ff876 with "arch-meson build --buildtype=debug -Dtests=false -Db_lto=false".

Earlier, performing the same swiping gesture caused a crash similar to bug 105160. I rebuilt from commit 5b29be3998 to pick up the fix for that in commit 01a633b6eb, and now I'm seeing this new assert instead.

More gdb info:
(gdb) frame 4
#4  0x00007f58e7f0e2f0 in tp_tap_handle_state (tp=0x5619dd09aab0, time=2162337668) at ../src/evdev-mt-touchpad-tap.c:1030
1030					assert(tp->tap.nfingers_down >= 1);
(gdb) info locals
_i = 0
t = 0x5619dd09ba60
filter_motion = 0
__PRETTY_FUNCTION__ = "tp_tap_handle_state"
(gdb) p *t
$8 = {tp = 0x5619dd09aab0, index = 0, state = TOUCH_END, has_ended = true, dirty = true, point = {x = 2960, y = 2860}, time = 2162337668, pressure = 72, is_tool_palm = false, 
  major = 0, minor = 0, was_down = true, quirks = {reset_motion_history = false}, history = {samples = {{time = 2162254619, point = {x = 2008, y = 2476}}, {time = 2162302736, 
        point = {x = 2960, y = 2860}}, {time = 2162337668, point = {x = 2960, y = 2860}}, {time = 2162224450, point = {x = 1694, y = 1398}}}, index = 2, count = 1}, 
  hysteresis_center = {x = 2960, y = 2860}, pinned = {is_pinned = false, center = {x = 0, y = 0}}, button = {state = BUTTON_STATE_NONE, curr = 0, timer = {
      libinput = 0x5619dd021580, timer_name = 0x5619dd09b330 "event11 (1) button", link = {prev = 0x0, next = 0x0}, expire = 0, timer_func = 
    0x7f58e7f0ffac <tp_button_handle_timeout>, timer_func_data = 0x5619dd09ba60}}, tap = {state = TAP_TOUCH_STATE_DEAD, initial = {x = 0, y = 0}, is_thumb = false, 
    is_palm = false}, scroll = {edge_state = EDGE_SCROLL_TOUCH_STATE_NONE, edge = 0, direction = -1, timer = {libinput = 0x5619dd021580, 
      timer_name = 0x5619dd09c2b0 "event11 (0) edgescroll", link = {prev = 0x0, next = 0x0}, expire = 0, timer_func = 0x7f58e7f11f47 <tp_edge_scroll_handle_timeout>, 
      timer_func_data = 0x5619dd09ba60}, initial = {x = 0, y = 0}}, palm = {state = PALM_NONE, first = {x = 1473, y = 1407}, time = 0}, gesture = {initial = {x = 0, y = 0}}, 
  thumb = {state = THUMB_STATE_MAYBE, first_touch_time = 2162224450, initial = {x = 0, y = 0}}, speed = {last_speed = 0, exceeded_count = 0}}
(gdb) p *tp
$9 = {base = {dispatch_type = DISPATCH_TOUCHPAD, interface = 0x7f58e8136860 <tp_interface>, sendevents = {config = {get_modes = 0x0, set_mode = 0x0, get_mode = 0x0, 
        get_default_mode = 0x0}, current_mode = LIBINPUT_CONFIG_SEND_EVENTS_ENABLED}}, device = 0x5619dd0976b0, nfingers_down = 1, old_nfingers_down = 1, slot = 0, 
  has_mt = true, semi_mt = false, arbitration = {in_arbitration = false, arbitration_timer = {libinput = 0x5619dd021580, timer_name = 0x5619dd09c250 "event11 arbitration", 
      link = {prev = 0x0, next = 0x0}, expire = 0, timer_func = 0x7f58e7f0a342 <tp_arbitration_timeout>, timer_func_data = 0x5619dd09aab0}}, num_slots = 2, ntouches = 5, 
  touches = 0x5619dd09ba60, fake_touches = 5, pressure = {use_pressure = true, high = 30, low = 25}, touch_size = {use_touch_size = false, high = 0, low = 0, 
    orientation_to_angle = 0}, hysteresis = {enabled = true, margin = {x = 20, y = 30}, other_event_count = 0, last_motion_time = 2162337668}, accel = {
    x_scale_coeff = 0.96024582293067029, y_scale_coeff = 0.64541112688782754, xy_scale_coeff = 0.67213114754098358}, gesture = {enabled = true, started = false, 
    finger_count = 1, finger_count_pending = 0, finger_count_switch_timer = {libinput = 0x5619dd021580, timer_name = 0x5619dd09c350 "event11 gestures", link = {prev = 0x0, 
        next = 0x0}, expire = 0, timer_func = 0x7f58e7f14814 <tp_gesture_finger_count_switch_timeout>, timer_func_data = 0x5619dd09aab0}, state = GESTURE_STATE_NONE, 
    touches = {0x0, 0x0}, initial_time = 0, initial_distance = 0, prev_scale = 0, angle = 0, center = {x = 0, y = 0}}, buttons = {is_clickpad = false, has_topbuttons = false, 
    use_clickfinger = false, click_pending = false, state = 0, old_state = 0, motion_dist = {x_scale_coeff = 0.024390243902439025, y_scale_coeff = 0.016393442622950821}, 
    active = 0, active_is_topbutton = false, bottom_area = {top_edge = 2147483647, rightbutton_left_edge = 0, middlebutton_left_edge = 0}, top_area = {
      bottom_edge = -2147483648, rightbutton_left_edge = 0, leftbutton_right_edge = 0}, trackpoint = 0x0, click_method = LIBINPUT_CONFIG_CLICK_METHOD_NONE, config_method = {
      get_methods = 0x7f58e7f10471 <tp_button_config_click_get_methods>, set_method = 0x7f58e7f10534 <tp_button_config_click_set_method>, get_method = 
    0x7f58e7f10582 <tp_button_config_click_get_method>, get_default_method = 0x7f58e7f10635 <tp_button_config_click_get_default_method>}}, scroll = {config_method = {
      get_methods = 0x7f58e7f0a971 <tp_scroll_config_scroll_method_get_methods>, set_method = 0x7f58e7f0a9aa <tp_scroll_config_scroll_method_set_method>, get_method = 
    0x7f58e7f0aa40 <tp_scroll_config_scroll_method_get_method>, get_default_method = 0x7f58e7f0aae1 <tp_scroll_config_scroll_method_get_default_method>, set_button = 0x0, 
      get_button = 0x0, get_default_button = 0x0}, method = LIBINPUT_CONFIG_SCROLL_2FG, right_edge = 5369, bottom_edge = 4301}, queued = 
    (TOUCHPAD_EVENT_MOTION | TOUCHPAD_EVENT_OTHERAXIS), tap = {config = {count = 0x7f58e7f0e72b <tp_tap_config_count>, set_enabled = 
    0x7f58e7f0e775 <tp_tap_config_set_enabled>, get_enabled = 0x7f58e7f0e7eb <tp_tap_config_is_enabled>, get_default = 0x7f58e7f0e869 <tp_tap_config_get_default>, set_map = 
    0x7f58e7f0e893 <tp_tap_config_set_map>, get_map = 0x7f58e7f0e8e9 <tp_tap_config_get_map>, get_default_map = 0x7f58e7f0e928 <tp_tap_config_get_default_map>, 
      set_drag_enabled = 0x7f58e7f0e937 <tp_tap_config_set_drag_enabled>, get_drag_enabled = 0x7f58e7f0e985 <tp_tap_config_get_drag_enabled>, get_default_drag_enabled = 
    0x7f58e7f0e9d7 <tp_tap_config_get_default_drag_enabled>, set_draglock_enabled = 0x7f58e7f0ea01 <tp_tap_config_set_draglock_enabled>, get_draglock_enabled = 
    0x7f58e7f0ea4f <tp_tap_config_get_draglock_enabled>, get_default_draglock_enabled = 0x7f58e7f0eaa1 <tp_tap_config_get_default_draglock_enabled>}, enabled = true, 
    suspended = false, timer = {libinput = 0x5619dd021580, timer_name = 0x5619dd09aa20 "event11 tap", link = {prev = 0x0, next = 0x0}, expire = 0, timer_func = 
    0x7f58e7f0e57f <tp_tap_handle_timeout>, timer_func_data = 0x5619dd09aab0}, state = TAP_STATE_IDLE, buttons_pressed = 0, saved_press_time = 2162224450, 
    saved_release_time = 2162302736, map = LIBINPUT_CONFIG_TAP_MAP_LRM, want_map = LIBINPUT_CONFIG_TAP_MAP_LRM, drag_enabled = true, drag_lock_enabled = false, 
    nfingers_down = 0}, palm = {right_edge = 5328, left_edge = 1612, upper_edge = 1308, trackpoint_active = false, trackpoint_listener = {link = {prev = 0x0, next = 0x0}, 
      notify_func = 0x0, notify_func_data = 0x0}, trackpoint_timer = {libinput = 0x5619dd021580, timer_name = 0x5619dd09c270 "event11 trackpoint", link = {prev = 0x0, 
        next = 0x0}, expire = 0, timer_func = 0x7f58e7f09545 <tp_trackpoint_timeout>, timer_func_data = 0x5619dd09aab0}, trackpoint_last_event_time = 0, 
    trackpoint_event_count = 0, monitor_trackpoint = true, use_mt_tool = false, use_pressure = true, pressure_threshold = 130, use_size = false, size_threshold = 0}, 
  sendevents = {config = {get_modes = 0x7f58e7f0be7e <tp_sendevents_get_modes>, set_mode = 0x7f58e7f0bf48 <tp_sendevents_set_mode>, get_mode = 
    0x7f58e7f0c012 <tp_sendevents_get_mode>, get_default_mode = 0x7f58e7f0c049 <tp_sendevents_get_default_mode>}, current_mode = LIBINPUT_CONFIG_SEND_EVENTS_ENABLED}, dwt = {
    config = {is_available = 0x7f58e7f0abef <tp_dwt_config_is_available>, set_enabled = 0x7f58e7f0abfe <tp_dwt_config_set>, get_enabled = 0x7f58e7f0ac54 <tp_dwt_config_get>, 
      get_default_enabled = 0x7f58e7f0ac9e <tp_dwt_config_get_default>}, dwt_enabled = true, paired_keyboard = {{device = 0x5619dd087580, listener = {link = {
            prev = 0x5619dd0875a0, next = 0x5619dd0875a0}, notify_func = 0x7f58e7f097b7 <tp_keyboard_event>, notify_func_data = 0x5619dd09aab0}}, {device = 0x0, listener = {
          link = {prev = 0x0, next = 0x0}, notify_func = 0x0, notify_func_data = 0x0}}, {device = 0x0, listener = {link = {prev = 0x0, next = 0x0}, notify_func = 0x0, 
          notify_func_data = 0x0}}}, key_mask = {0 <repeats 12 times>}, mod_mask = {0 <repeats 12 times>}, keyboard_active = false, keyboard_timer = {
      libinput = 0x5619dd021580, timer_name = 0x5619dd09c290 "event11 keyboard", link = {prev = 0x0, next = 0x0}, expire = 0, timer_func = 
    0x7f58e7f0964f <tp_keyboard_timeout>, timer_func_data = 0x5619dd09aab0}, keyboard_last_press_time = 0}, thumb = {detect_thumbs = false, threshold = 0, 
    upper_thumb_line = 0, lower_thumb_line = 0}, quirks = {nonmotion_event_count = 0}, lid_switch = {listener = {link = {prev = 0x0, next = 0x0}, notify_func = 0x0, 
      notify_func_data = 0x0}, lid_switch = 0x0}, tablet_mode_switch = {listener = {link = {prev = 0x0, next = 0x0}, notify_func = 0x0, notify_func_data = 0x0}, 
    tablet_mode_switch = 0x0}}

# ./libinput list-devices
Device:           Power Button
Kernel:           /dev/input/event3
Group:            1
Seat:             seat0, default
Capabilities:     keyboard 
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           Video Bus
Kernel:           /dev/input/event4
Group:            2
Seat:             seat0, default
Capabilities:     keyboard 
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           Power Button
Kernel:           /dev/input/event0
Group:            3
Seat:             seat0, default
Capabilities:     keyboard 
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           Lid Switch
Kernel:           /dev/input/event2
Group:            4
Seat:             seat0, default
Capabilities:     switch
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           Sleep Button
Kernel:           /dev/input/event1
Group:            5
Seat:             seat0, default
Capabilities:     keyboard 
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           Chicony USB 2.0 Camera: Chicony
Kernel:           /dev/input/event7
Group:            6
Seat:             seat0, default
Capabilities:     keyboard 
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           HDA Intel PCH Mic
Kernel:           /dev/input/event9
Group:            7
Seat:             seat0, default
Capabilities:     
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           HDA Intel PCH Line Out
Kernel:           /dev/input/event10
Group:            7
Seat:             seat0, default
Capabilities:     
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           AT Translated Set 2 keyboard
Kernel:           /dev/input/event5
Group:            8
Seat:             seat0, default
Capabilities:     keyboard 
Tap-to-click:     n/a
Tap-and-drag:     n/a
Tap drag lock:    n/a
Left-handed:      n/a
Nat.scrolling:    n/a
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   none
Click methods:    none
Disable-w-typing: n/a
Accel profiles:   n/a
Rotation:         n/a

Device:           SynPS/2 Synaptics TouchPad
Kernel:           /dev/input/event11
Group:            9
Seat:             seat0, default
Size:             107x59mm
Capabilities:     pointer gesture
Tap-to-click:     disabled
Tap-and-drag:     enabled
Tap drag lock:    disabled
Left-handed:      disabled
Nat.scrolling:    disabled
Middle emulation: n/a
Calibration:      n/a
Scroll methods:   *two-finger edge 
Click methods:    none
Disable-w-typing: enabled
Accel profiles:   none
Rotation:         n/a