I had a similar experience on AA on Thanksgiving Sunday. The replacement crew that they thought they found at ORD was actually already illegal and in their hotel. They had to find another crew and get them back to the airport. An extra two hours delay wasn't fun on top of the initial two that we were at when we started boarding.

My understanding is that those situations mostly happen when there is a delay in getting data into the crew scheduling system so the updates that would cascade through to reflect that the pilot is going to still be illegal for their next flight haven't propagated. I don't know if that is always the case, but it is what we were told.