Some people have reported that after updating to Mac OS X 10.6.5, while playing video they have been suddenly and forcibly logged out and returned to the login window. When this occurs, all applications are quit and unsaved work is lost.
For those affected, this behavior is random. It has been reported to happen when playing different types of video using a variety of players including Flash player, VLC, QuickTime Player, iTunes, and Bridge. Often times the problem occurs immediately after entering full screen video mode.
Rumor has it that this will be fixed by the forthcoming Mac OS X 10.6.6 update. Until then, the only workarounds are trying to use a different version of your media player, or turn off hardware acceleration features of the player if that’s an option.