Regarding the cancelled UA976 on 1/18, which I unfortunately was on:

Around 11p, a second plane was located and we had to wait about 20 minutes for it to be warmed up and for the crew to arrive. Once we were authorized to board, we were urged over the PA to cooperate to expedite the process, as they were running dangerously closed to hitting their FAR work limits. That's exactly what happened, though, and we had to deplane. We had to wait a bit for Chicago to decide whether or not they'd create a new segment in the morning, but they opted to have everyone placed on the next available flight instead. Interestingly, the crew spent a solid 15 minutes playing musical chairs with pax to get standbys seated together. It seems odd that they would burn time doing this (I didn't hear any activity below indicating cargo was still being loaded) rather than getting us into the air.