A few years ago on reserve I was called out to DH to Boston. When I got to the gate they had to pull 2 off to get me and another DHer on the flight. I wound up in a seat next to the business partner of the one of the guys pulled. He was not happy with me. I told him I understood but was just doing what the company told me to do.
If I ever find myself in that situation again I may just let the flight push without me and tell sked it was gone when I got to the gate after seeing this.
BTW in my days at Republic j4j their crew sked would give you a dh and never get seats for you. You'd have to rely on the agent or call sked at the gate to solve the problem. By that time they were already boarding.
Didn't have this problem once I returned to us air however since the changeover to AA ops I've had this happen three times. And I'm not on reserve being called out to dh. These are DH flights built into a regular line. I've had to call from the gate each time. That's the last time I'm doing that. From now on if I show for a dh and there is no listing for the flight I'm going to sit there and call sked after the push.