A couple of months ago, I flew UA from SJC to DEN, connected UA DEN to CLE, then CO to PVD. I was on a CO ticket, since it was cheaper. UA upgraded me from SJC to DEN. When I tried to board the connecting flight in DEN (to claim my exit row seat), I was told that I had been booted off of the flight since I was a no-show in SJC. I showed them my BP from that flight. I still had to go back to the GA and get a new BP, obviously no longer in the exit row. I never got an explanation for this, but I suspect it might have had something to do with CO issuing the ticket.