Once a fellow interpreter told me a great story about meeting a deaf couple for the first time. At the end of the conversation the deaf couple signed "nice to meet you". At the same time the terp was thinking "nice to see you" but signed "nice to meet you" using the two finger hand shape for "see" instead of one finger for "meet"... Her hands were in a more horizozntal position as well... You can figure out the rest!!!
I think this is the single most common blunder. I've heard this one so many times. Never gets old, though.