Man, I hate false awakenings. I would not infrequently have them in middle school and high school: I would dream about waking up ridiculously early, going to school, doing all the tests, suffering through the classes I didn't like, spending literally hours on the bus going to and from, and then I would wake up shortly before the bus came at 6:30AM and think to myself oh come onnnn...
Yvain has a pretty good story on his blog, too.
This is a kind of "X files" thread.
Post experiences which spooked you, which made you doubt reality, mathematical or physical laws, your sanity, memory or perception. The more improbable the better, but no second-hand legends please, share only what you personally experienced. If you had the event later explained rationally please use rot13 to avoid spoilers.