I was (technically still am) on Flight 859 this morning from ORD to SFO. We had a mechanical issue that sent us back to the gate and caused a flurry of rebooking on to other flights (we were set to leave at 7:00 and the next flight at 8:00 was apparently very light). We took off about 8:45, but because of all the rebookings, there were five passengers on the entire flight. Once the doors were shut, the FAs upgraded the entire cabin. It's a good thing my upgrade cleared on a UDU. I'd have felt a wee bit cheated if I'd used an SWU or a CR1.

That said, it may have been the most surreal flight I've ever flown, one FA for every 1.25 passengers.