I got stuck on Nemesis at Alton Towers recently.....Twice. The first time, we had just finished our ride and were waiting to enter the station, but the car in the station had a problem.
The second time, we just left the station and were heading for the lift hill but the tyres didnt seem to engage with the car properly and we never made it to the lift hill. It took about 20 minutes for them to try....and fail....to fix it and then we were let off.