At United we have senior manning. As a junior guy moving across two fleets in three domiciles over three years, half on reserve, I have never once been asked to perform additional flying past trip termination. It just doesn't happen. It could, in theory, but there's always a senior guy happy to answer the phone for a quick buck. Last-minute mandatory reassignments are such a serious QOL hit they should be as rare as their only fair causes, hurricanes, floods, etc. And even then they should be compensated well. They are here, in case it should matter.