LOL! I can still remember enough Spanish from when we lived in Spain to occasionally startle folks who assume I couldn't possibly speak the language. My favorite incident was in a Mexican restaurant where two bus boys were making comments about the clients. I was amused, but not drawing attention to myself, since I didn't care that much about what they were saying.

However, when they said something really rude about a rather attractive lady there, I gave them the full-on stare with raised eyebrow... which is when one of them elbowed the other hard and muttered sotto voce, "Shut up, you moron! She can understand us!" The weird thing is, the other guy actually started to argue with him -- until he realized I'd just nodded agreement with the first guy.