I hear all these stories of people getting ejected from coasters and falling to their deaths. That is scary. Nothing more scarier than being on a ride wondering if your seatbelt or harness is working properly.
What even more disturbing are the 16 year old operators who secure you into the rides who do their jobs lackadaisically with a "I am so bored I could care less about any safety precaution because I'm only making $6.50/hr" attitude.
It's not like this in most parks, actually. You'd have a higher chance to be hit by a car or a falling piano from the sky than falling off a rollercoaster. But it does suck to hear about these accidents for they shouldn't happen at all.
I almost fell off of the Cyclone at Six Flags in Atlanta. Chalk it up to a skinny guy having a big girl best friend in high school, and the seat belt popped off midway through the ride. Funny thing was, she yelled that it happened, and the next thing I know, I'm wrapping my leg around the lap bar and holding on for dear life. I almost didn't want to let go even after the ride had stopped.
(Needless to say, got over my fear of roller coasters really quickly after that.)
Decapitation is the one that surprises me. I don't know if whole trains ever go off the track.
The scariest coaster I've ever been on is also the most famous:
The Cyclone.
The first reason it's scary is because it has some wicked turns. The Cyclone was built in an era before anyone knew about modeling or even much about the physics of roller coasters. What holds you onto the track is brute steel, not centrifugal force. The Cyclone has a particularly vicious sting close to the end you don't expect but will certainly remember.
The second reason it's scary is because it doesn't look all that well maintained. Sure it's a national landmark but it looks like a museum piece. The cars are ancient, the seats are worn, the bar is burnished smooth from decades of riders, it squeaks and rattles and shudders, there's paint chipping from the scaffolding, and the operators look like homeless guys picked off the street for a day's work. It's definitely shabby and you wonder just where your train will land if it went off the track as you get thrown around against the car with only this one rather small bar holding you in place. I think this makes The Cyclone more frightening than any modern roller coaster because the new ones all seem so shiny, new, and safe with their all-steel tube, computer-designed curves and hills and their safety harnesses.