I hate it when I'm about to do something, and then I forget what it was that I was about to do.
For example, I was in the bank the other day and the teller said, "What can I do for you today?" and I was like "Oh Crap. What did I come here to do?. Then I looked down at the gun and ski mask I had and I suddenly remembered: "Oh yeah!"
"I was wondering if I could lock in my mortgage rate," I said. But when I left the bank I still felt like I was forgetting something.