Exactly that happened to me a few times. Could not use the last sectors of RTWs or normal return tickets.
Did not hear anything from anybody about this. Miles for flights taken have been credited.
Usually i buy my tickets in Europe - and there are customer protection laws or regulations in place in most countries which prohibit the airlines of charging extra in that situation.
Customers in the US are far less protected, so at least in theory the airlines could do something, but i never heard about such a case. Anybody experienced such a problem?