Ok, So here are the only VERIFIED accidents that have happened at Alton Towers according to the Database where I got my information (officail sources confirm any of this) And you'll be pleased to know that not ONE of them is due to ride failure, more to Stupidity!
An Air ambulance was called to Nemesis, it wa'snt clear why but they stopped the ride. It's beleived it was merely someone who needed an ambulance and completely unrelated to the ride.
Then on the 30th of May a young man stepped into a restricted area on nemesis and was struck by a car travelling at station speed! And that's it. There have been other accidents but not to do with the rides, well not as such. For example, a couple of idiots threw some coins into a car of Nemesis when it was at high speeds and cut a womans face, another one is that (probably the same idiots had it been the same day) threw stone at the people on Air when it did the "corkscrew" part of the track. Another woman is currently trying to sue Alton Towers because she say when the AIr brakes stopped on Black Hole her restraint bar was'nt locked and shot her to the front of the carriage breaking her ribs! Hmmm.... so, Alton Towers 1 - Liars trying to make money - NIL!
Something similar to that happened at pleasureland, southport on truamatizer, the train was pulling into the station when a ride op was walking up the stairs at the end of the station and was hit by the train. He landed on the floor of the station and you could see him while going by on the train. Was pretty funny when we found out he was ok. And on the subject of harness failure, while i was on the ride at that time, we got into the station and my harness wouldnt open, probably because i had it in too tight, well it took about 5 mins to get it open again, not very serious. Last time i was there i seen that the stairs arent there anymore :P