My personal favorite was on the ICL PWS-400. The ICL PWS-400 was a custom hardware design built by ICL. I was on the team of 5 (two from Microsoft, three from ICL) whose job it was to port MS-DOS 4.1 to this new hardware. The cool thing about the PWS-400 was that it had some custom hardware that allowed real mode applications to access bank switched memory in 4K pages. This allowed apps to run in the background without impacting running applications.
Since the five of us we were the entire development team, we also did a lot of ad-hoc testing. One of my personal favorites was running a game that Valorie had brought me from school. I'm not sure which game it was now, but every time I played it, when it got to a specific spot, the machine would spontaneously reboot.
We put the machine under an ICE (in circuit emulator - a hardware tool that lets you see what's going on inside and outside the CPU) and discovered that the CPU was being externally reset. That ruled out some wierd software bug.
The hardware guys took the game and the machine and started looking.
After a couple of days, they came back to me and announced they'd found the problem. It turns out that the trace on the motherboard for the PC speaker was too close to the trace on the motherboard for the CPU reset line. When you played a specific sound on the PC speaker, EMF emissions from the speaker trace would cause the CPU reset to go high, which caused the CPU to reboot.
Gotta love working with hardware 🙂