DTW on a NW flight, roughly 6 months after NW moved to the new terminal. We tried to push back (this was at one of the gates where planes were pushing back on engine power, rather than using a tug). Engine revs, we go nowhere. Engines back to idle, rev up again, no movement. Captain comes on the PA and says that the parking brake is stuck. So we wait for maintenance to show up. They show up, say nothing is wrong with the parking brake.

Captain comes back on the PA and says that it's apparently not the parking brake, but that we've created a rut in the concrete, and they're going to need to get a tug (from the other side of the airport) to push us out of the rut.