Saw a runaway Beagle Pup about thirty years ago - started in the proper manner by the key, two people in the cockpit.
Presumably the throttle was fairly wide open and the brakes were off; it shot forward, wingtip clipped a petrol pump (or something similar) on the corner of a hangar which spun it round the pump and into the side of the hangar. It was very briefly heading my way and about thirty feet way... ulp!
Right wing clearly wrinkled, not sure about the prop and engine. I believe it was a couple of mechanics who were intending to just taxy it elsewhere on the airfield.