When I stay up too late I am often bewildered by my alarm clock when it wakes me up, unable to figure out what the numbers mean for a while. Nonetheless there must be a part of me that knows what's going on because I always end up setting it again so I have enough time to get ready after sleeping some more.
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.