Okay, I'll admit to my recent one. I went out with a deaf friend who's Indian, and he ordered Indian food, so I explained that I don't like it because it's too spicy. So he showed me the sign for spicy, which he does as a 5 hand coming from under the chin while wiggling the fingers.
Next time we went out, there were a few of us, and I was asking him how to say "peppers", and my friend responded by moving a 5 hand with wiggling fingers horizontally to the side....
Needless to say I got laughed at a lot for thinking that was the sign.