I have one but's very brutal, is that alright?
There were many moments I had something like that, especially from Switzerland where one needs to learn two languages at the same time, but nothings comes into my mind but a brutal one from a nightmare.

I remember I was with my friend when some ask how old he was in Spanish. When meaning to say that that he was seven teen he accidentally said that he liked to eat anus. 
It was more awkward that he said it to my aunt and she has slight homophobia.