Today, most hardware has some protection against abuse, but in the days of the BBC micro it was common for hardware and os to completely trust the software. There were plenty of storys of damaged monitors because of invalid timing parameters.
One of my own storys, and I'm a bit hazy so some details are probably wrong: We found an old computer in a university dumpster some night, and decided to mess with the floppy drive. More beer than common sense around. There was some way to tell the hardware which track to read/write, with sane values from 0 to 79. But it was a byte , we could go to 255, so we decided to go up up up!
Well, the drive did exactly as commanded. 80 - 85 worked quite well, except the floppy wasn't guaranteed to be magnetically coated. But once we pushed it too far, the read head went literally over the edge: It jumped off the axle, dropped on the spinning disk below, got a serious yank, and the tiny wires got snapped off.
All of this with a single x86 out instruction, I think in dos 3.x debug. The OS was not stopping you if you did something stupid, there was no hardware protection or anything.