Overspeeding a big diesel can be catastrophic, and humorous in a macabre kind of way. I have an account that builds high wheel AG-sprayers with hydrostatic wheel motors to propel the vehicle at road speeds nearing 40mph. These are the AG sprayers on near eight foot tall tires. I was back in R&D one visit and they had a Cummins 500hp diesel on the shop floor, that had suffered an overspeed event. They were testing new wheel motors, but on the road test, decel down a long grade, the momentum of the vehicle was actually causing the wheel motors to act as pumps, which drove the main hydro pumps, which spun the diesel, to well over 3,000 rpm, and......BANG!

Head was off the engine and I looked in the bores and started laughing. One piston, what remained of it, was jammed, sideways in one bore. In another bore the piston was, well, gone, and the remains of the con-rod small end had whipped around in the cylinder. The oil pan was full of schrapnel.