I was just starting to learn English at the time, but will never forget this really embarrassing mistake.
We were learning about body parts. The English teacher was asking each student to stand up and show a particular body part. When he came to me, he said: Show me your face. But, I heard the French word fesses, not the English word, face. I looked at him and asked, Really? He said yes. So I stood up, bent over, and showed the class my bottom. Fesses in French = buttocks. And, how unfortunate for me, but it very much sounds like the English word, face.
The class gave me the nickname Buttface after that.
– Emanuele, QC, Canada