I've had 2 runaways
1 was when I let my nephew drive a Nitro MT1 I built out of a box of parts I bought of some dude labled spares. Got a whole truck in it
Anyways my nephew clipped a curb, and the throttle arm popped off the motor. along with the steering linkage from when he clipped it. thing went flying down the street till it hit a pothole and luckily the hit of the pothole veered it into a curb instead of a car.

2nd was when I put a failsafe on my MT2... the fail safe caused me to lose control, and bust an a-arm... Failsafe went in the garbage after that. Now I run without any fail safes and not had a single run away since