On a system with the VIA p4M900 graphics chipset, booting X with the -vesa driver causes a freeze.  Here are two logs from after different freezes:

http://launchpadlibrarian.net/11994451/Xorg.0.log (feb 08)
http://launchpadlibrarian.net/12856714/Xorg.0.log (mar 08)

Backtraces didn't reveal any obvious error, but here's what the reporter collected:

"After installing and updating Hardy, the X server continues to run and
gdb can be attached to the process:

(gdb) bt full
#0 0xb7faf410 in __kernel_vsyscall ()
No symbol table info available.
#1 0xb7df973d in select () from /lib/tls/i686/cmov/libc.so.6
No symbol table info available.
#2 0x081b2225 in WaitForSomething (pClientsReady=0xbfe4c810)
    at ../../os/WaitFor.c:236
        client_priority = <value optimized out>
        client_index = <value optimized out>
        highest_priority = <value optimized out>
        i = <value optimized out>
        waittime = {tv_sec = 94, tv_usec = 176000}
        wt = (struct timeval *) 0xbfe4c7d0
        timeout = <value optimized out>
        clientsReadable = {fds_bits = {0 <repeats 32 times>}}
        clientsWritable = {fds_bits = {137307688, 137307905, -1075525912,
    137307680, 137292440, 136163204, -1075525880, 137301840, -1209573044, 32,
    -1209687072, -1210494255, 137307688, 136163204, -1075525752, 134725985,
    137307688, -1075525800, 1, 137297328, 137307688, 48, -1209687072,
    135860875, 137284224, -1209577484, -1209573056, 137301848, -1075525768,
    -1210477600, -1209573056, 137301848}}
        curclient = <value optimized out>
        selecterr = 104
        nready = <value optimized out>
        devicesReadable = {fds_bits = {-1209573024, -1075525988, -1209687172,
    135372057, 0, 136443240, 136440576, 137307680, 137307688, 24, -1209687072,
    135372215, 137307688, -1210477600, 1, -1208302704, 136443240, -1209577484,
    -1209573056, 137292440, -1075525928, 137307840, -1209573056, 137292440,
    137307688, -1209577484, -1209573056, 137307688, -1075525896, -1210477600,
    -1209573056, -1210494255}}
        now = 90164
        someReady = 0
#3 0x0808d69d in Dispatch () at ../../dix/dispatch.c:425
        result = <value optimized out>
        client = (ClientPtr) 0x82efdb8
        nready = -1
        start_tick = 0
#4 0x0807471b in main (argc=11, argv=0xbfe4cd44, envp=0x0)
    at ../../dix/main.c:452
        pScreen = <value optimized out>
        i = 1
        error = 136163204
        xauthfile = <value optimized out>
        alwaysCheckForInput = {0, 1}

"And here is what happens after the kill of the X server:

(gdb) cont
Continuing.

Program received signal SIGUSR1, User defined signal 1.
[Switching to Thread 0xb7c8ba30 (LWP 5617)]
0xb7faf410 in __kernel_vsyscall ()
(gdb)
Continuing.

Program received signal SIGTERM, Terminated.
0xb7faf410 in __kernel_vsyscall ()
(gdb) bt full
#0 0xb7faf410 in __kernel_vsyscall ()
No symbol table info available.
#1 0xb7df91d8 in writev () from /lib/tls/i686/cmov/libc.so.6
No symbol table info available.
#2 0x081bcabe in _XSERVTransSocketWritev (ciptr=0x82ec850,
buf=0xbfe4c784,
    size=1) at /usr/include/X11/Xtrans/Xtranssock.c:2192
No locals.
#3 0x081bb72f in _XSERVTransWritev (ciptr=0x82ec850, buf=0xbfe4c784,
size=1)
    at /usr/include/X11/Xtrans/Xtrans.c:914
No locals.
#4 0x081b6666 in FlushClient (who=0x82eeb50, oc=0x82ec150,
extraBuf=0x0,
    extraCount=0) at ../../os/io.c:1060
        before = <value optimized out>
        remain = -1075525756
        i = 1
        len = <value optimized out>
        oco = (ConnectionOutputPtr) 0x8317670
        connection = 10
        trans_conn = (XtransConnInfo) 0x82ec850
        iov = {{iov_base = 0x8319b48, iov_len = 32}, {iov_base =
0x8092eb8,
    iov_len = 137292624}, {iov_base = 0x20, iov_len = 3219441640}}
        written = 0
        padsize = 0
        notWritten = 32
        todo = 32
        padBuffer = "\000\000"
#5 0x081b7054 in FlushAllOutput () at ../../os/io.c:812
        index = <value optimized out>
        mask = 0
        oc = (OsCommPtr) 0x82ec150
        client = (ClientPtr) 0x82eeb50
        newoutput = <value optimized out>
#6 0x0808d915 in Dispatch () at ../../dix/dispatch.c:524
        result = 136353016
        client = (ClientPtr) 0x82eeb50
        nready = 0
        start_tick = 0
#7 0x0807471b in main (argc=11, argv=0xbfe4cd44, envp=0x8092eb8)
    at ../../dix/main.c:452
        pScreen = <value optimized out>
        i = 1
        error = 136163204
        xauthfile = <value optimized out>
        alwaysCheckForInput = {0, 1}