Had two experiences at Hollywood (MGM) Studios. got to test ToT before it opened. It was actually a VERY uncomfortable experience. We got through the line and pre-show just fine. Oh wow! We got to look around the boiler room. Then we walked in, sat down, and pulled down our lap bars. The doors closed...and we sat there. Sat there a bit longer. Then we sat some more. Eventually the doors opened again and the CM explained the ride wasn't working and we would have to board another elevator. This was frightening because we were about the ride an attraction that is supposed to simulate something going wrong...and it just did...for real. Now, we got into the other one, went up to the 5th dimension, went up, and the doors opened just fine. Then we fell and got off.
I still remember being frightened about the ride might have just broke for us.
Oh yeah, got to test RnR too. However, there isn't an exciting story for that one...