A couple years ago, I was starting my dad's 1959 Wasp with an MC-10 on it.
Apparently, the throttle was stuck open, cause it shot across the yard into my trailer.
The frame kinda folded a bit in front of the seat. I had to cut the front frame rails off.
The trailer fared better, but needed some sheet metal work.
Not one of my better days.
Not a big fan of direct drive because of what happened in this video which probably happens quite often plus the gearing limitations do not really promote running on a large track along with high engine wear due to high rpm.
A stuck throttle as Terry mentioned is pretty much the same thing as what happened in the video because the clutch will lock up so its always a good idea to have someone at the 'brakes' just in case.
A few years ago Frank Weir and myself experienced a runaway kart and though things miraculously worked out and I was able to stop the kart it was a very frightening and dangerous thing that could of had grave consequences.
Was at a dirt race & a guy started his kart to warm it up. Throttle stuck, he grabbed the nerf bar and it about screwed him into the ground going in circles. Took a bit to get a few people to get it stopped and shut off.