One of my friends went to America and didn't realise you should tip bar staff. (Rather uncommon in the UK.) At some point during one evening the topic came up, for whatever reason - and he was forced to admit that he hadn't been leaving any tips at all! Cue such a degree of shock from his American hosts that he felt moved to talk to the bar person about it when he next went to get drinks.

"Oh, don't worry", said the bar guy. "I spotted your accent straight away, and I know English people never leave tips."

Now perhaps what my friend should have done at this point was take him at his word and continue not to leave him any...