Originally Posted by MSPeconomist

The problem with this reasoning is that an airline could just cancel segments whenever a flight is oversold and thus avoid being responsible for either IDB or VDB payments. Earlier in the day the OP had a ticket and reservation; the OP apparently checked in for both segments without any problems and at that time had both boarding passes. It certainly should not be the OP's problem or responsibility that the GA canceled the segment when this shouldn't have happened and moreover the GA failed to notify the OP that something bad had been done to the OP's PNR/ticket, so that the error wasn't discovered until the OP tried to board the second segment.