Steps to reproduce on a laptop with a multitouch screen (in my case, a Lenovo X1 Carbon Gen 2):
1. Lock the desktop.
2. Start lightly tapping away on the laptop screen and triggering various bogus multitouch events.
3. The Xorg issue reported in https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-input-evdev/+bug/1121379 will eventually trigger and gnome-session will crash
4. You are able to view the unlocked desktop, and have limited keyboard/mouse access to the desktop.
The issue seems to be similar to https://bugs.launchpad.net/ubuntu/+source/unity/+bug/1308572 , which is marked fixed, but it looks like in the event of a crash the screen can still become unlocked.
Relevant logs:
==> /var/log/auth.log <==
Aug 2 09:14:10 SOMEHOST compiz: PAM unable to dlopen(pam_kwallet.so): /lib/security/pam_kwallet.so: cannot open shared object file: No such file or director
y
Aug 2 09:14:10 SOMEHOST compiz: PAM adding faulty module: pam_kwallet.so
Aug 2 09:14:10 SOMEHOST compiz: pam_succeed_if(lightdm:auth): requirement "user ingroup nopasswdlogin" not met by user "SOMEUSER"
==> /var/log/syslog <==
Aug 2 09:14:24 SOMEHOST gnome-session[2065]: WARNING: App 'compiz.desktop' respawning too quickly
Aug 2 09:14:24 SOMEHOST gnome-session[2065]: WARNING: App 'compiz.desktop' exited with code 1
Aug 2 09:14:24 SOMEHOST gnome-session[2065]: WARNING: App 'compiz.desktop' respawning too quickly
==> /var/log/Xorg.0.log <==
[ 445.600] (EE) Backtrace:
[ 445.600] (EE) 0: /usr/bin/X (xorg_backtrace+0x48) [0x7f6921bd8c78]
[ 445.600] (EE) 1: /usr/bin/X (0x7f6921a30000+0x7d3e7) [0x7f6921aad3e7]
[ 445.600] (EE) 2: /usr/bin/X (0x7f6921a30000+0x138845) [0x7f6921b68845]
[ 445.600] (EE) 3: /usr/bin/X (0x7f6921a30000+0x138357) [0x7f6921b68357]
[ 445.600] (EE) 4: /usr/bin/X (0x7f6921a30000+0x13afa2) [0x7f6921b6afa2]
[ 445.601] (EE) 5: /usr/bin/X (0x7f6921a30000+0x15c1d4) [0x7f6921b8c1d4]
[ 445.601] (EE) 6: /usr/bin/X (mieqProcessDeviceEvent+0x1cd) [0x7f6921bbb0cd]
[ 445.601] (EE) 7: /usr/bin/X (mieqProcessInputEvents+0xf7) [0x7f6921bbb1e7]
[ 445.601] (EE) 8: /usr/bin/X (ProcessInputEvents+0x9) [0x7f6921ac2fe9]
[ 445.601] (EE) 9: /usr/bin/X (0x7f6921a30000+0x55802) [0x7f6921a85802]
[ 445.601] (EE) 10: /usr/bin/X (0x7f6921a30000+0x5994a) [0x7f6921a8994a]
[ 445.601] (EE) 11: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xf5) [0x7f691f56cec5]
[ 445.601] (EE) 12: /usr/bin/X (0x7f6921a30000+0x44e7e) [0x7f6921a74e7e]
[ 445.601] (EE)
[ 445.724] (EE) BUG: triggered 'if (!(event->device_event.flags & (1 << 5)))'
[ 445.724] (EE) BUG: ../../dix/touch.c:644 in TouchConvertToPointerEvent()
[ 445.724] (EE) Non-emulating touch event
[ 445.725] (EE)
[ 445.725] (EE) Backtrace:
[ 445.725] (EE) 0: /usr/bin/X (xorg_backtrace+0x48) [0x7f6921bd8c78]
[ 445.725] (EE) 1: /usr/bin/X (0x7f6921a30000+0x7d3e7) [0x7f6921aad3e7]
[ 445.725] (EE) 2: /usr/bin/X (0x7f6921a30000+0x138845) [0x7f6921b68845]
[ 445.725] (EE) 3: /usr/bin/X (0x7f6921a30000+0x138357) [0x7f6921b68357]
[ 445.725] (EE) 4: /usr/bin/X (0x7f6921a30000+0x13afa2) [0x7f6921b6afa2]
[ 445.726] (EE) 5: /usr/bin/X (0x7f6921a30000+0x15c1d4) [0x7f6921b8c1d4]
[ 445.726] (EE) 6: /usr/bin/X (mieqProcessDeviceEvent+0x1cd) [0x7f6921bbb0cd]
[ 445.726] (EE) 7: /usr/bin/X (mieqProcessInputEvents+0xf7) [0x7f6921bbb1e7]
[ 445.726] (EE) 8: /usr/bin/X (ProcessInputEvents+0x9) [0x7f6921ac2fe9]
[ 445.726] (EE) 9: /usr/bin/X (0x7f6921a30000+0x558c6) [0x7f6921a858c6]
[ 445.726] (EE) 10: /usr/bin/X (0x7f6921a30000+0x5994a) [0x7f6921a8994a]
[ 445.726] (EE) 11: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xf5) [0x7f691f56cec5]
[ 445.726] (EE) 12: /usr/bin/X (0x7f6921a30000+0x44e7e) [0x7f6921a74e7e]
[ 445.727] (EE)
[ 450.837] (II) AIGLX: Suspending AIGLX clients for VT switch
I'm surprised an X crash leaves enough of X running for you to interact with it; that seems highly surprising.