A bit of an odd occurence with my CS5.5 (trial) installation: It seems to be corrupting videos slightly enough that it can no longer read them, resulting in a "Media Offline" error message when reopening the project. The video files are still there, but will not play back in the Source Monitor. However, they will play in VLC Media Player, though it picks up that the .AVI file is broken and needs to be repaired for playback, and it does.
If I copy the files again from my SD card and repoint the project to them, they will load. However, they will simply be broken again upon restart of the program.
If it matters, the video files are 720p 25fps videos from my Pentax K-r.