Was on my second year as an apprentice and my boss had left me to fit an engine speed controller to a truck with a big PTO powered pump (diverts the drive shaft of the vehicle to drive the pump),
After I had it all fitted up, I walked up the person who I was introduced to when I arrived there, asking if it was safe to start the vehicle and if they could test it (the had to flick a switch and change gear, I preferred them to do it, as I would be listening to what the motor was doing, trimming in hunting, and the response to load changes)
So he starts up the truck, walks off and talks to someone for like 20 minutes, with me just chilling in the passenger seat, then comes back and we begin the test, He sets the gearbox to PTO, and puts it into the correct gear, straight off the bat, It doesn't sound correct, and seems to be getting worse by the second, I ask him to take it out of gear as it was really bad by that point, he said "No, this is how it normally sounds", and left it in gear,
He then began to hold the RPM increase button for our controller, raising to 2200RPM, I began smelling burning hydrocarbons and being young I was following his lead, but my every instinct was to shut it down, this continues for a few minutes, I'm silently freaking out at the sounds of this truck not sounding OK at all,
About 5 or so minutes pass, and...
"CRACK-sssssssssssss", The cab of the truck shudders hard, the engine revs way up, trips our controller due to RPM being too high and returns to Idle, He turns off the truck, I'm very frightened,
Well the gearbox had just exploded, In a brand new truck, Spraying super-heated hydraulic oil onto the concrete of there workshop which was now making cracking noises of its own, and pop corning,
Turns out the workshop had closed off the inlet and outlet valve of the pump, so the PTO and gearbox where driving an almost locked pump, And just heated up more and more until things went bad.
Fun times... because I was not at fault. Just scared out of my mind.