Imagine that Person A is bi/tri/whateverlingual, and Person B eventually shows interest in learning whatever other language(s) they know. After a few basic lessons, B starts to get the hang of it, and starts to use the language(s) more casually. One day when A and B are doing a regular thing for them (whether it be washing dishes or watching a movie), B tries to say something completely innocent, but messes up the pronunciation so much they accidentally say something incredibly dirty.