When working FreeCAD 0.13, especially after clicking OK in the preferences window, KDE often crashes very badly. All desktop effects become disabled, windows are unmovable (even with Alt+drag), all panels and window decorations disappear. Yaquake is opened automatically. Windows from all virtual desktops are moved to the same one. Desktop background stays intact, right-click menu works. So far the only way I've found to return to normal operation is to reboot.
It appears kwin has crashed. Do you get any hint for a crash, e.g. in .xsession-errors?
No, .xsession-errors has no valuable information. The last thing I see in there is FreeCAD's ASCII splash screen. But the problem is easily reproduced by starting FreeCAD, going to Edit > Preferences, clicking Display on the left side, the clicking OK. This happens with an up-to-date Ubuntu plus the FreeCAD 'daily' builds, currently 0.13-git201203140150~oneiric1. Is there anything else to check?
Looking at the FreeCAD home page, the latest stable version is 0.12. Does this issue also appear with that version?
I have no chance to try 0.12, but I tried it with 0.11.4446 from the official Ubuntu Oneiric repsitories, and it has the same issue, reproducible in the exact same way. I guess it also happens with 0.12.
It seems like it's not a KDE/KWin issue. It also crashes TWM, so the problem might be on a lower level, most likely in X. Will you help me further diagnose the problem?
Sorry, but the KDE bug tracker is not a support forum. I am very certain that the OpenGL mode causes the graphics driver to crash. Please ask in the forum of your graphics driver vendor how you can debug this issue.