I've submitted a PR for this. http://www.freebsd.org/cgi/query-pr.cgi?pr=124638
If I start any composite window manager ( compiz, kwin4 + desktop effects, metacity + composite enabled ), and then start any OpenGL application (glxgears, stellarium, ...), the X would crash immediately.
I've found nothing useful in the log files.
PS. I've seen other people reporting similar issue in other threads.
PPS. There's no problem with driver 169.12. There are problems with 173.*, 177.*