A very, very bad thing to do. Most often, when a bad move is committed, many people are made very unhappy. Short of the rape, torture, and murder of the Pope's favorite Sunday school pupil, a Bad Move is the worst possible thing to do.
Subtracting from one side of an equation and not from the other, suggesting that someone go outside wearing a black belt with brown shoes, or trying to bribe the ravenous Bugblatter Beast of Traal out of swallowing you whole when you only have 12 Altairian dollars to give it. All of the above are sufficient Bad Moves.