Me too! Though one time when I was little, I dreamed I was flying on a blue velvet cushion - straight into the mouth of my little plush fox, who had grown a lot bigger. What made it even scarier was that I knew I was dreaming, but I couldn't wake up.
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.