Basically any time you get onto a roller coaster there's something in the back of your head that wonders if you're going to come flying off the ride when you're on it. Considering how many times the rides operate on a daily basis and how infrequently you hear about things like that actually happening, there's very little chance that something goes wrong.
But, things do happen. Someone films a roller coaster that is apparently somewhere in China, and it appears to be a somewhat tame one at that. No 100+ foot drops, no inversions and it doesn't look like it gets that far off the ground.
Regardless, the train comes down from a drop and picks up speed as it goes around a turn. The turn doesn't look like it has any bank to it and the train comes flying off the rails. The video cuts out pretty shortly after you first see the train coming off, so it's unknown if and/or how badly anyone was injured. I love roller coasters but man it's tough to hear about every time something like this happens.