Getting all the way to the movie theater before realizing I forgot my money. Going back home (20 min each way) and being too annoyed & disgusted to go back. Maybe tomorrow.
That sucks. I always have this fear every time I'm about to walk into the theater that I've actually got no cash on me. So far, it's all been irrational but I know one day I'll walk in, go to buy a ticket and open an empty wallet.