I was flying RDU-IAD-ORD, and due to overbooking, volunteered to take a direct (slightly later) flight on AA to ORD (it was one of the last UA flights anywhere that day, and I think the last AA flight), got free ticket according to the deal. The original flight left and we (me + 3 other people) went to the AA gate where we were told that UA "pulled back" our tickets and they won't issue BP. Both gate agents (UA & AA) were not too helpful - we had to run between the gates because they couldn't figure a way to call each other. It was worse than it sounds, as basically we forced the UA agent to issue us ptix to use on AA, only like 2 minutes before the AA gate closed and we had to sprint back to the gate to catch it. It took me like 20 minutes to calm down afterwards, as it was a really close call.

Is anyone able to tell me - what went wrong? We all felt cheated by the UA, because after offloading us they basically left us to fend for ourselves... Is there any point in escalating this issue further?

Well - They obviously didn't just leave you to fend for yourself...and you did get on the flight...but if you want to write a letter, go ahead.

Hmm... I seem to disagree. The UA agent was telling us "the AA agent has to simply pull the reservation" and sending us back to AA agent, who was saying that UA is not releasing our tickets to them, and that we should go back to the original gate. If not for us insisting on paper tickets at the very end (due to advice from AA agent), we would probably be left at RDU for the night...

It could also be that the problem are UA agents, who don't know how to properly endorse a ticket to another airline!
I had the same problem 4 times at different airports (BOS,IAH,ORD and DTW) this year (different trips , endorsing to CO), everytime the issue was that UA agents hadn't done their part correctly and I had to run back to find an UA CSR to re-do the whole process.

Well, I guess I might think different if I was actually there.

Also, which concourse was the UA gate in? That walk must have been terrible

It was the same concourse as AA, so the walk was doable within 4-5 minutes normal speed, faster while running :-P

I've had similar problems with other carriers being able to see/pull/use the etix issued by UAL checkin/gate agents due to irrops. Based on these and other people's experiences, it seems that a systemic training error may have occurred. Perhaps they need to re-assess and re-train?

Oh, I thought you WERE in ORD, meaning the walk would have been a much less doable 20 minutes

I was booked DEN-DFW yesterday and gave up my seat due to an oversold situation. I was booked on an AA flight arriving 3 hours later.

When I went over to Concourse C to get a boarding pass, the American Airlines GA said United didn't ticket it properly. The GA had to call their Planning department who didn't know why United didn't complete the ticketing either. The GA however did get permission to "just book him" and I got my boarding pass.

I did miss Channel 9 and E+, but I still don't know what to do with an even more restricted DBC Free voucher (can only book V, Q, or W)...

I have always encountered some sort of problem, when being endorsed between the two. Either the ticket does not "push" properly or they cannot find the ticket number, etc. The best remedy I have found is to ask the GA or TA for the ticket number and they will print a copy of the itin with the new ticket number on the bottom, or as you found just ask them to do it as a paper ticket and take the flight coupon over to the other counter.