My coworker's complete inability to spell is insanely annoying! I can only overlook "must of", "Ur", "suppose to" and various other murderings of our language for so long. English is her first language, she was born and raised in the United States, and she has a Bachelor's degree, so I know she was taught how to spell!
Another coworker is driving me crazy because she doesn't know certain things about the new system we have (or hasn't used that function in so long that she's forgotten what to do) but insists on "helping" people out when they have questions. Two of us in the office are the best resource as far as basic functions within the system because it's all we work with during the day - we have no special projects or anything else to take our time away from using this system. However, this coworker still thinks she knows best, even though by her admission she hasn't had anything to do with most of the system since January.