This one doesn't involve a client but it's too awesome not to share. One day my boss called me to come and help her with a problem she was having with her computer. Well, hm, it turned out that the issue she was having was that everything she typed was coming out IN CAPITAL LETTERS.
Yeah, you can guess what the cause of this problem was. The best part? There were TWO OTHER PEOPLE in her office at the time who SHE was training on our computer system.