I've always classed an accident as something that isn't intended to happen.
Lets say that a branded vehicle (or a vehicle that is supposed to be allocated) has developed a problem in the depot or breaks down during the day, the depot has no choice but to send another bus out as a replacement. Even if it is branded for another service. Since the breakdown is unintended, i'd see the wrong allocation in this instance to be an accident.
Arriva North East: Latest News & Discussion - February 2015