Bust

A bust is a negative status ailment which exists in Final Fantasy XI exclusively for Corsairs. When a corsair uses his Phantom Roll ability and then uses Double-Up one or more times and ends up with a number greater than 11, he will receive this status ailment. The status ailment gives a moderate negative effect of the same type that would have been granted should the corsair have gained a positive relative to the type of roll attempted. The penalty is not given to all of the party, as the bonus would have been, only the corsair.

Example
If the corsair rolls a bust while doing Chaos Roll, which would normally give him an attack bonus, he will receive a penalty to his attack score.