KWin in KDE4 has a shortcut to toggle compositing on and off. Sometimes (not always) when I toggle compositing off with this, X crashes. Dunno if this is a bug in kwin or X, so I'm reporting it in both places. Attaching the X log. There's a not-too-useful backtrace near the end.
Created attachment 20179 [details]
A workaround for this was just committed to kwin: http://bugs.kde.org/show_bug.cgi?id=174782#c1 (includes what looks like more info about the bug itself)
Created attachment 20310 [details]
backtrace of the server crash
Created attachment 20311 [details] [review]
avoid calling DRIGetDrawableInfo with NULL pScreen
This patch adds an extra check for a NULL pScreen pointer into glxdri.c getDrawableInfo(), in line with an existing check for a NULL pDraw pointer.
The patch is a hack and won't always help I'm afraid. The problem is that pDraw is a stale pointer.
This should be fixed in xserver 184.108.40.2061.