This was tested on F18.
Xvnc: rrmode.c:265: RRModeInit: Assertion `num_modes == 0' failed.
1. Start an Xvnc server - for example:
$ /opt/thinlinc/libexec/Xvnc :1 -query localhost -geometry 1024x768 -depth 16 -ac +extension Composite passwordFile=~/.vnc/passwd
2. Connect to Xvnc server, e.g.:
$ /usr/bin/vncviewer localhost:1
3. Launch a Gnome Shell session on the Xvnc server. e.g.:
$ DISPLAY=:1 dbus-launch gnome-session --session=gnome
4. Click around a bit, launch some things, move them around and wait for the crash.
It is actually simpler than that. Just starting Xvnc and xterm is enough to provoke the bug.
What happens is that RandR checks that everything is cleaned up on a server reset. Apparently we are not doing that properly.
Fixed in r27444.
Could not reproduce using xterm in an Xvnc session. Closing.