i have, many times.
Air at Alton Towers, the restrasints kept getting stuck closed on one of the trains. we managed to get right at the front!!! only problem was, we got stuck on it for at least half an hour after :\
Colossus at Thrope Park. with the restraints, they werent locking properly. stood there for 3 and a half hours to get on it.
Samurai at Thorpe Park. once again, restraints woulndt close for some of the rides. we sat down, the bars came down, the ride was about to start and all the bars were released automatically.
Ramesis Revenge at Chessington. got stuck on it (restraints, bloody hell, lol) while it was set to one of its extreme routines, nearly threw up over the girls in front of me.
madame tussauds seems to be having some problems, or the theme parks just hate me