My dad has a story about that. He was talking with a friend who had a Yamaha 250MX (this is before the YZ was created) and they were about to take off on a ride. He kicks the Yammie a few times, then it goes "kerBANG!" and then starts running normally. Weird. Well, it wasn't that loud of a bike so they exchange a few words about where they're going. The guy pulls in the clutch, leans his chin over the front fender, revs the bike up and dumps the clutch for a big impressive roost. The bike shoots backward with a big impressive roost, throwing him over the bars instantly and leaving the bike to flop over onto the ground. Total time from clutch-popped to everything being still was about half a second. Apparently, the bike kicked back with enough force to start the motor backward. Being a two-stroke, which doesn't know or care if it's running forward or back, everything seemed normal after the first kerBANG.