AA's system does not seem to be error proof. I was flying DEN-DFW-TPA, with a tight DFW connection, and since I got to DEN early I went standby on an earlier DEN-DFW flight (didn't want to do an earlier DFW-TPA flight because I was getting picked up). I cleared standby and got to DFW a few hours early.

When I tried to board my TPA flight, my boarding pass was rejected because I was a no show on my originally booked DEN-DFW. They luckily had one extra seat left, so I made the flight with no problem. When I got to TPA, I called AA to reinstate my remaining leg, because it was also cancelled.

It's not like I did anything that strange to trigger this chain of events. I never thought to ask the agent when I cleared standby "can you make sure my original flight is cancelled?" - but I guess I might do that next time.