I know someone living in the Czech Republic - he made quite an effort to learn the language.
He went with his girlfriend to her grandparents place - they spoke no English. One of her sister's daughters, about five years old was staying there at the time. The child showed him a picture she was working on. He said, in his pigeon Czech, "You have a nice picture".
Unfortunately, several things conspired against him.....
1, He couldn't recall "obraz", the word for picture.
2, He just filled in by using the English word "picture".
3, He pronounced "Picture", as most native English-speakers do, as "pitcher".
4, "Pitcher" sounds exactly like the Czech word for something that the Forum will not let me type in.
So, he effectively stated to the child that "You have a nice chuff!" ( as the Forum would Bowdlerise it)....
He knew, from the stunned silence, that he had done something, but it was hours before he could get her alone and find out what it was he'd done.....