Let's see I guess it would have to be the huge lack of common sense that people seem to have. It seems to be an epidemic where I work. This frustrates me more than any of the other ones. Another one would be asking a question over and over in different ways to try to get answer, sadly this is what I get from the grown ups in my world at work. If the answer is something you don't like rearranging the words it will not change the answer to something that you want it to be.
I am sure there are more but these are the the biggest ones for me. What is your biggest one? Do you have to deal with it regularly?