A year ago, I was going to the local Institute of Mathematics (I live in Bucharest) to attend a short talk on mathematical logic. The talk was scheduled at noon. Given that I had spent the night before at my girlfriend's and we were going somewhere together in the afternoon, I took her with me. While walking towards the Institute, I said to her that I don't remember the name of the speaker. She said that maybe it's a guy that we had met at a conference two months before (that conference was on a completely different area of math, namely algebraic combinatorics). She didn't have any prior knowledge of the logic talk or of that guy's mathematical interests. As we entered the room, we saw that it was really that guy. I still can't explain that..
Possibly you'd previously mentioned his name to her before forgetting it. Or she'd seen the name somewhere. Or she'd seen him on the street.
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.