At one point in my long and illustrious college career, I thought I wanted to be a sign language interpreter. My very first American Sign Language class in college was taught by a Deaf man, Dennis -- who also happened to be gorgeous. New language. New culture. Distracting professor. Perfect.
As with any language, small nuances are all that separate one word or sign from another. A slight change in the orientation of one's hand, a raised eyebrow instead of a lowered one, or a tiny crook in a finger can completely change the meaning of what you're trying to convey.
And, as with any language, the things that stick with us most easily and permanently are the...uh...socially-unacceptable terms and phrases. The ones that could get you arrested, or even slapped in the face, if used in front of people fluent in the language.
Naturally, someone managed to expose me to a few of these, right before our midterm exam.