I was on Test Track when it broke down, me and my brother were in the car, on the right hand side, we had just gone outside and the car was moving at full speed round the first loop, as we were going down the part where the big digital screen gives you your speed, the car began to slow down. We were moving at about 2mph past the crowd in the queue below. Then the car reached the part of the track where it raises up on the right hand side to circle the building. As it slowly got steeper it became more painful for us because the seatbelts were cutting into us. We reached where it becomes steepest and then the car stopped! We had all been hoping that it wud keep going so we would become flat again. Anyway, we were there for 5 minutes or so, but i amused myself by getting the people in the car with me to shout 'help!' to the crowd below who were gathering round to look at out stranded car.
When we got off the ride eventually they gave us a free ride, then shortly after, i found that my parents who were in the car behind (we were singlke riders who managed to get in the same cars) had been sat on the straight part of the track just outside!
Thats the only major breakdown i've experienced, but spaceship earth stops a lot!