Originally Posted by
milepig
But, back to A-B-C-B-A. In theory, walking away from the last segment (B-A) is against the terms of the ticket and they can come back to get you, but the odds of this happening seem low.
I'm sure this varies in other countries, but my understanding is that in the US the airlines cannot legally "come back to get you." That is, they can ask you to pay some fare difference, but they have no legal means to enforce this (and if they tried to sue you, would lose). I vaguely remember the airlines attempting to lobby congress for laws allowing them to collect fare differences in these situations, and being shot down. I could be wrong though, would appreciate if anyone knows the law on this for sure. And of course this doesn't prevent an airline from coming after a travel agent by threatening to no longer sell tickets to that person if they don't pay up.