Have you ever wrongly assumed someone didn't speak your language? What happened?

Saw a similar question on reddit that had interesting answers.

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Not me but having worked at a university I’ve heard of men who spoke their language thinking they were having a private conversation about the woman who was also on an elevator and when the elevator stopped at the floor to let the woman off they were talking about she said “Have a nice day” in their language.....both men red-faced.