This happens to me all the time
. In fact, I just got back from a recitation today in which I had a bit of difficulty understanding the material. I called my TA over and asked him a question about the lab we were working on. He started to explain something (while I started to daydream), and then I immediately blurted out the correct answer. My TA kind of looked at me funny, then said, "OK, that's completely correct, but why?" and I just gave him a big old
I notice this tends to happen a lot when I'm extremely tired (for example, yesterday I had about three hours of sleep). All I need now is to convince my professors to get rid of that pesky "show all work" requirement on exams, and I'll be set.