I know this is not necessarily a flight from hell, as most of it went pretty smoothly, but we did experience a delay that I found funny so I wanted to share.
I was flying from PHX to MSP, so it was a large plane and it was booked full. Boarding took quite a long time, as people sometimes seem pretty unaware of others around them and just take their sweet time.
We had just gotten everyone boarded when the flight attendant came on the intercom and announced that we would all have to get off the plane, and take everything with us. The plane we were originally supposed to be on had maintenance issues, so they had swapped planes. No big deal, except that this plane had come straight from the hangar, and apparently had to go through an extensive security sweep before we could board. The crew had not finished this sweep, but apparently no one had told the gate agents that before they let us all on the plane.
The sweep was finished in about twenty minutes, and then we had to go through the entire drawn-out boarding process again. All in all, we only ended up taking off about forty minutes late, and made up most of the time in the air. Not a terrible inconvenience, but certainly an odd story (and makes me wonder about procedure and communication if we were all allowed to board before the sweep was finished!).