I have a co worker who can be the nicest person one day, and walk in the next day to be the biggest you know what on the face of the planet. It makes for an extremely difficult work environment. It seems as if her anger is only pointed toward me and my boss, because when she talks to people in person or on the phone she is as sweet as can be.
On a good day, she can be a blast to work with, but the bad days definitely outweigh the good, even if they are less frequent than her good days. I'd say out of the month, there are usually 3-4 crappy days for her. They are usually grouped together.