My husband and I know a bit of ASL and in college would sign to each other a lot. The sign for the letter F is the same as the colloquial "OK" sign in English. So, when one day at a crowded table I asked him how he did on a test he had, and he replied with a forced grin and the "F" sign, everyone else thought he was saying "A-Ok" and only I caught his real meaning.
That was a fun, intentional use of a mixed meaning of a gesture...but we can get in trouble with our gestures when traveling when we assume that the gestures we use mean the same thing everywhere. They don't. This article goes over some you might want to avoid when traveling abroad (and this one you can share with your kids, cause they don't get into details, just say that this is an "offensive" or "crude" gesture in such and such country).