One of my favorite pieces of debug folklore was the story of a minicomputer or some other serious machine rebooting in the middle of the night. I can't remember exactly but it was in some university during the 70s/80s. An important server would spontaneously reboot in the middle of the night on some days but not all.
Finally, someone sat up all night to watch what happens and observed a cleaning lady unplugging the machine from power in order to plug in a vacuum. When she was finished, she plugged the computer back in and left.
Another on of my favorites, not bug but more malicious, was someone wrote a program to swing the heads on one of those old giant hard drives back and forth. At the right frequency, the back and forth momentum would cause the drive to walk away from the wall and eventually unplug itself.
We had a similar one when I worked for a large supermarket chain. We had one store that their In-Store Processor (ISP), which was a big IBM 6F1 RS/6000 box (about the size and looks of a large mini-fridge) was plagued with all kinds of weird and sporadic errors. Code that works fine in the rest of the chain would only sometimes work there, all kinds of disk errors, slowness, etc. We sent technicians to the store multiple times and they could never figure out the issue, we even swapped out most of the hardware.
Eventually, we discovered that the young girl that was responsible for changing all the prices was using the server room as her makeshift office. She would stick all her magnetic clipboards with the upcoming price changes to the side of the server. When she heard the tech was coming she would clear out all her stuff since she knew no one was supposed to be in there. The magnets were wreaking havoc on the disks causing the errors. The store manager got her some proper office space and the errors went away.
Re: " someone wrote a program to swing the heads on one of those old giant hard drives back and forth. At the right frequency...cause the drive to walk away"
On some TRS-80 models I discovered a certain POKE command would make the screen flash, click, and "bounce" the image, similar to a power spike. So in the university lab I wrote up a small BASIC script with a delay timer to repeat "DANGER: Computer Overheating!" as the clicking/flickering increased in rate. I stood back to watch and waited for the timer to kick in. Nearby students would freak out. When the script was done, it deleted itself in case they got the lab assistant to come over. I was a stinker.
That was the port that switched the screen from 64 columns to 32. A different bit in the port also controlled a physical relay that was used to switch the cassette tape recorder on and off. Toggling the pair together quickly did indeed look and sound quite dramatic - and would probably knacker the relay if left for long enough.
Finally, someone sat up all night to watch what happens and observed a cleaning lady unplugging the machine from power in order to plug in a vacuum. When she was finished, she plugged the computer back in and left.
Another on of my favorites, not bug but more malicious, was someone wrote a program to swing the heads on one of those old giant hard drives back and forth. At the right frequency, the back and forth momentum would cause the drive to walk away from the wall and eventually unplug itself.