The general wisdom is that if you do this "once in awhile" the airline won't ever say anything - in fact, I once had a CS agent tell me to do it. When I asked "what about the last segment since I'd already be checked in for the flight", the response was "just get off the plane at airport B and keep walking."

In the olden days, my travel agent used to actually get cheaper fares by booking A-B-C-B-A and then cancelling the B-C-B portion of the ticket, this kept the original fare. But, I think the airlines have long since caught onto this little trick. I know she was always a bit worried that the "overnight audit" would catch it, but I never had a refare happen.

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. As others have said, skipping B-C will almost certainly cause the cancellation of the rest of the ticket.