Not really a *scary* moment for me, more of a *shiver* moment:
One summer (forget which now) I rode the King Cobra like 20 times in a row.
There was no line and we could get off and get back on. (It was a weekday)
Anyway, about an hour later we heard a loud noise, and go over to see that the Cobra was stuck in the helix. We later found out that one of the wheels came off and it ground into the helix to stop.
No one was seriously injured, though. But it made me think twice about the King Cobra, even though I *LOVE* that ride and miss it.