Ah, the days before memory protection seemed necessary...
VAXen have a block-move assembler instruction MOVC3, which works much like the C function memcpy(). I remember trying to debug a program that would fall over occasionally, and when it did the resulting memory image made no sense. I eventually found that I had the arguments to the MOVC wrong, and when executed it shifted my entire executable program address space by 8 bytes. After that none of the symbols matched, and the debugger had no idea what was where.
At least that would only have affected my local program memory space, VAX/VMS having good memory protection. Way back in Uni we used ICL systems with a home-grown OS. It had an 'ALTER' command that worked much like a POKE in BASIC, and would allow you to read/change locations in your local address space, you could pause a running program & poke around in it. There was also an assembler instruction which would allow you to send a command to the OS from within a program, a la "system()" in modern C. Both useful, but no-one had tried combining them until one of my fellow students did. Turned out that the system() function was treated as I/O, and handled asynchronously, the command was queued and the program paused until it was executed, then resumed with the result of the command. Of course, on a busy system a paused program could be swapped out and another scheduled until the async operation completed. If you passed an ALTER command like that, since the system didn't have virtual memory, when the ALTER command was run there was no guarantee that the address it was changing belonged to the calling program any more...
My fellow student ran his program, and shortly afterwards every terminal in the room hung. Odd, he thought, but crashes weren't uncommon. We went for coffee, and when we returned the system was back, so he tried again. Same result... At that point he had the wit to call the computer centre and say "umm, I think that might have been me", so the end result was a thank-you from the sysadmin and credit for finding a bug, and not any punishment for crashing the main undergrad system twice in an hour.