Not really life-and-death scared.
Once when I was on a roller coaster, the ride started, but the grab bar, which was supposed to lock in the down position on our laps, didn't lock, and was free to loosely move back and forth.
I didn't think there was serious chance of actually getting thrown out of the roller coaster, but we did hang on very tightly, and the guy next to me was saying Hail Marys, so yes, it was somewhat scary.