I guess it was when I said " would you like some cock' instead of 'would you like some coke" to a member of my host family in the UK...  That was mortifying

I never forget it. My tutor ask me "How old are you" and I say twelve instead twenty :D We were laughing for one hour and when we finished, we were laughing again.

When i was in 6th grade I said '' I m sex grade// in front of my English teacher :) 

One of my roommates at university was from El Salvador and I traveled there with her to visit her family. While we were there, we attended her cousin's civil wedding ceremony. At the reception after the ceremony, one of her other relatives was talking to me and asked "Has estado en [word I couldn't understand]?" In English that's, "Have you been to....?" I had him repeat himself at least four times before I finally figured out he was asking if I'd ever been to the the US state of Mississippi, but thestronly accented way he pronounced 'Mississpipi' threw me off.  Incidentally, I had never been there, so it was a bit of an anti-climax once I finally figured out what he was saying and answered. I hope he didn't think I was completely ignorant of the names of  the states in my own country!

This isn't my own experience, but it was certainly embarrassing. It happened in an English language class that was discussing hopes and desires for the future. The teacher asked the question 'What do you want most in life?'.

A teenage French girl put her hand up and tried to say 'I want happiness'.

Unfortunately, that was not what the teacher or the rest of the class heard. French people have difficulty pronouncing initial 'H' sounds, so her answer sounded like

'I want a'peeniss'

So you can imagine what everyone thought that she had said!!