There are three circumstances under which my brain just locks up:

1. When I'm caught by surprise. If someone suddenly walked up to me and said, "Quick! How much is 8 times 7?" or "What is your middle name?" I'd say, "uh...uh...uh..."

2. When there is a lot of peripheral noise/motion, such as after church with everybody milling around chatting, kids playing, etc. Someone will be asking me a question, and I know I'm supposed to respond, but I just lock up.

3. When I least expect it.