A few years ago, I was on Air after the park had closed. There were only 4 of us on the last train and it stopped for a second on the way up then for about 5 minutes above the Nemesis track. The staff at Nemesis just laughed at us!
There was another time that Duel broke down. I was scared the whole time, not because it was Duel or felt unsafe but because we were near the end, I expected a member of staff to sneak behind us in the dark then jump out at us (experience it before at a different theme park), it didn't happen, got a brilliant high score though as there were millions of targets and only two of us shooting. That was a day of break downs for me as Nemesis broke down just before I got on and Oblivion broke down on my third time round. As we were going up, the chain lift sounded different but we made it to the top and back into the station, when I ran back round for my fourth turn (obviously the park was dead, and I would have got back on the train i got off), the ride attendant announced Blivy had technical difficulties.
Other than that, there were minor breakdowns (stuck on the break run of Oblivion for 20 mins, which was annoying as they could have undone our harnesses and let us walk off the cat walk) and a power cut which stopped half the park (Nemmys half). That day was funny as we weren't on a ride when the power cut happened, The RMT was an inch from the station when it stopped (on that really horrible corner where one person gets squashed to death) and there were people begging for other people to throw food to them as the were stuck for about an hour. Although we did need a drink and couldn't buy one coz the staff couldnt access the tills (didn't realise till after that X-Sector was still running, along with most of the park). Many people left after getting money back but AT did stay open an hour later and we paid to get in the next day for a fiver(ish)