I was looking for a wallet that had fallen onto a very cluttered floor. When casual inspection failed, I started clearing the clutter-- then found the wallet in the middle of a cleared space. The most plausible explanation is some sort of attention failure, but it wasn't a small wallet.
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.