I was on my way to the market today when I heard the people behind me say gweilo ("white person". Well, technically, "white devil"). I couldn't understand what else they were saying, but looked back and they were looking at me. So I walked a bit slower, and as they passed me I said (in Cantonese) "I understand Cantonese."
I didn't have a clue in hell what they were talking about, but they looked horrified. Very satisfying.