I suddenly remembered what was my surreal story, perfect for rationalist creepiness :)
It was I think 1999 or so, and one afternoon it began to rain strange, white filaments from the clear sky. It seemed like cobweb, only that they were a lot thicker and the threads were about two feet long. I was in a large parking lot of a supermarket, and the rain covered it completely. It went on for some minutes, and I was utterly amazed: the strangest thing though was that nobody paid attention to the strands rain. I was running around trying to collect sample of that substance, but as soon as you touched it, it shrinked to almost nothing. A fistful of that sticky thing, if kept in one hand, would reduce to almost a black grain of sand.
I finally managed to collect a sample by wrapping the substance around a thick pen. I kept it in the fridge for about a week, unchanged. But I didn't know anyone who could perform an analysis, so in the end I threw it away.
I still don't have any explanation for the phoenomenon, it has never happened before and I've never witnessed it after.
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.