Multiple bad disk cables (more common in IDE era, but happened once with SATA). Interestingly enough, Windows would reduce the drive speed on certain errors, so I had a drive that booted up in UDMA/133 and the longer it was running the slower it got, eventually settling in at PIO mode 2. Switching the drive cable fixed it.
A sound card that wasn't screwed in to the case, so if you pushed the phone connector in too hard it would unseat. I still don't know how that happened; it must have been me (unless someone pranked me) but the sound-card hadn't been changed in like 2 years at that point.
A DIMM wasn't fully clipped in, but the system worked fine for weeks until someone bumped into the case.
Things that were actually intentional:
We expect anything plugged in externally (e.g. USB, ethernet, HDMI) to be plugged and unplugged without needing to restart the system. This sounds banal, but wasn't always the case. I had a network card with 3 interfaces (10BASE5 AUI, 10BASE2 BNC, 10BASE-T modular plug) and you needed to power off the system and toggle a DIP switch to change which was in use.
I've seen server and minicomputer hardware with hotpluggable CPUs and RAM
Eurocard type systems (e.g. VME, cPCI) could connect all sorts of things, and could run without restarting. This sort of blurs the line as to what a "node" is. If you have multiple CPUs on the same PCI bus, is that one node or many?
eGPUs have made hotplugging a GPU something that anyone might do today. If you run this setup, then the majority of the computational power in your system can appear and disappear at will, along with multiple GB of RAM.
Multiple bad disk cables (more common in IDE era, but happened once with SATA). Interestingly enough, Windows would reduce the drive speed on certain errors, so I had a drive that booted up in UDMA/133 and the longer it was running the slower it got, eventually settling in at PIO mode 2. Switching the drive cable fixed it.
A sound card that wasn't screwed in to the case, so if you pushed the phone connector in too hard it would unseat. I still don't know how that happened; it must have been me (unless someone pranked me) but the sound-card hadn't been changed in like 2 years at that point.
A DIMM wasn't fully clipped in, but the system worked fine for weeks until someone bumped into the case.
Things that were actually intentional:
We expect anything plugged in externally (e.g. USB, ethernet, HDMI) to be plugged and unplugged without needing to restart the system. This sounds banal, but wasn't always the case. I had a network card with 3 interfaces (10BASE5 AUI, 10BASE2 BNC, 10BASE-T modular plug) and you needed to power off the system and toggle a DIP switch to change which was in use.
I've seen server and minicomputer hardware with hotpluggable CPUs and RAM
Eurocard type systems (e.g. VME, cPCI) could connect all sorts of things, and could run without restarting. This sort of blurs the line as to what a "node" is. If you have multiple CPUs on the same PCI bus, is that one node or many?
eGPUs have made hotplugging a GPU something that anyone might do today. If you run this setup, then the majority of the computational power in your system can appear and disappear at will, along with multiple GB of RAM.