When running the 'event-debug' tool with an Intuos Pro, the message "libinput error: libinput bug: invalid event, no fingers are down" will appear several times after placing a fifth finger on the touchpad.
Running the tool with GDB, it appears that the "if we have too many touches" case inside 'tp_unhover_touches' (evdev-mt-touchpad.c) is satisfied and subsequently clears out all active touches once the fifth finger goes down. This is because "nfake_touches" should be zero at this point due to the kernel switching out BTN_TOOL_QUADTAP for BTN_TOOL_QUINTTAP (which isn't handled by libinput).
This error can be delayed until 6 fingers are in contact by handling BTN_TOOL_QUINTTAP, but it isn't really a solution (that's the last *TAP tool reported by the kernel, and the touchpad can report far more). Libinput needs to sense when the *TAP tool "overflows" and rely on slot state instead.
fwiw, this error message can also be triggered by the kernel re-using a slot immediately without a EV_SYN in between, see the patch for Bug 88903 here: https://patchwork.kernel.org/patch/6123751/
Author: Peter Hutterer <email@example.com>
Date: Thu Apr 9 14:23:16 2015 +1000
touchpad: detect fake finger overflow after BTN_TOOL_QUINTTAB