Nothing can screw up a well-managed network faster than the people for whom you built it. Whether it's user error, optimistic expectations, or simply that bastard Murphy, IT's job is rarely predictable.
Lucky for you, there are lessons to be learned from others' misfortunes. So rather than wait to make your own forehead-shaped dent in the office wall, familiarize yourself with the screwups detailed below. It will make you that much more prepared to safeguard your IT environment from the ever-evolving boneheaded tendencies of those you serve.