I work for a Cat dealer and repair and build truck engines and we occasionally see the aftermath of an over speed or runaway. It is always cool to see the damage that can be done. I had one that was a 15L engine that made over 2100 lbft of torque runaway because of a turbo failure and went into a self shutdown. A piston came threw the side of the block and went all the way around several times. I think it was #3 cylinder (of an inline 6 cylinder) when I removed the cylinder head the block was then in 2 pieces. The rod and piston cut the engine block in half.
I have seen others that were pushrod engines where a rod would hit the camshaft and break it in half and push it 3 or 4 inches out side the block. Then there are those with multiple cylinder that fail from over speeds and will have several holes in the block. The last one we did had 10 or 11 holes in the block. That was as close to an engine grenading as you can get.
This is what the big torque truck engines look like C-16 Cat 600hp 2100lbft torque (plus

). I just completely rebuilt this one for a logging customer that hauls heavy (120,000 to 160,000 pounds and sometimes more):