As a non-native speaker this happened to me when I worked as a senior house officer ( I think it is called an intern in the USA) in Walsall, UK.

In conversations with my friends there I had learnt that bollocks is another word for testicles. So one day, when I had to examine a man who had suffered a few kicks in his groin and crotch area I said to him in the presence of a couple of his friends "Now I would like to examnine your bollocks." When everyone doubled up with laughter I realised bollocks isn't used in polite conversation.