My best mistake: As an Anglophone speaking French, when I don't know a word, I just say the English word in a French accent. It quite often works. This time i was at the paediatrician with my 2 week old daughter. While he was doing the check up, he said to me "Comment vous nettoyez le nez de votre bébé?" (How do you clean your baby's nose?)
If you don't have kids, you might not even know what one of these is anyway. In English it's called a nasal aspirator. In French, as I now know, it's called a mouche-bébé.
So I said to the doctor in my best French accent, "Avec l'aspirateur."
Which, for those of you who don't speak French, means "with the vacuum cleaner."
He did look horrified. Luckily he stopped dialling social services when I started frantically saying trying to mime what I really meant.
How about you? Have you ever embarrassed yourself in a foreign language?