When using KDEs "kwin" and "plasma", as well as several other applications, Xorg crashes in some configurations. This has been reported on https://bugs.kde.org/show_bug.cgi?id=179480. I've also been able to reproduce this when using "Xvnc", on several different systems. Here's a copy of my traceback:

Core was generated by `/home/astrand/ctc/vnc/./tightvnc/unix/xserver/hw/vnc/Xvnc :50 -fp unix/:7100 -p'.
Program terminated with signal 11, Segmentation fault.
#0  0x08216c12 in miHandleValidateExposures (pWin=0x893bc60) at miwindow.c:453
453                 if (REGION_NOTEMPTY(pScreen, &val->after.borderExposed))
(gdb) bt
#0  0x08216c12 in miHandleValidateExposures (pWin=0x893bc60) at miwindow.c:453
#1  0x081fa92f in UnmapWindow (pWin=0x8a7e0c8, fromConfigure=0) at window.c:3062
#2  0x08169532 in compFreeClientWindow (pWin=0x8a7e0c8, id=143) at compalloc.c:206
#3  0x08165224 in FreeCompositeClientWindow (value=0x10100, ccwid=65792) at compext.c:93
#4  0x081ef83a in FreeResource (id=143, skipDeleteFuncType=0) at resource.c:561
#5  0x08168a40 in compUnredirectWindow (pClient=0x892d028, pWin=0x8a7e0c8, update=0) at compalloc.c:256
#6  0x08166736 in compChangeWindowAttributes (pWin=0x8a7e0c8, mask=64) at compinit.c:128
#7  0x082167ff in miCheckSubSaveUnder (pParent=0x8a7e0c8, pFirst=0x1, pRegion=0xbfa9d560) at miwindow.c:226
#8  0x082166e4 in miCheckSubSaveUnder (pParent=0x893bc60, pFirst=0xbfa9d560, pRegion=0xbfa9d5a0) at miwindow.c:198
#9  0x082168d7 in miChangeSaveUnder (pWin=0x8bb6980, first=0x8a3a778) at miwindow.c:273
#10 0x081f9e83 in MapWindow (pWin=0x8bb6980, client=0x89874c0) at window.c:2806
#11 0x081d2464 in ProcMapWindow (client=0x89874c0) at dispatch.c:695
#12 0x081d88d3 in Dispatch () at dispatch.c:454
#13 0x081ec490 in main (argc=7, argv=0xbfa9d7d4, envp=0x10100) at main.c:441

Since this crash has been reproduced on different systems, both with Xvnc and a normal Xorg, as well as with multiple applications, I believe this is a generic Xorg bug.