I made an error when reassembling a system on my test bench. I had connected the ATX power supply "first" without verifying that it was switched off. While inserting the Slot 1 CPU, the system spontaneously sprang to life.
I was worried about possible damage to the board, the CPU, or other components, and started testing using my most reliable coffee-spill CF card. I got to the BIOS screen, but it would fail to boot into DOS. I tried another CF card for Windows ME. which booted, but the system started freezing later while testing a PCI card. It seemed similar enough to the DOS boot problem that I thought this could be an indication damage somewhere. I even tried to boot DOS from a floppy, and had the same problem with DOS freezing at boot.
I spent the next couple of hours troubleshooting, swapping the CPU for an identical one, trying a different CF adapter, using an IDE adapter, swapping out the motherboard for an identical one (to keep everything consistent), etc. etc.
I then tired booting the CF card in a laptop and it worked fine. So the CF card was fine, but not in this desktop board. And every part tested the same when I swapped it out. I tried booting to gparted to look at the drive partition, but it was shown as unformatted. It booted fine in the laptop, but Scandisk complained about being unable to read the last block and that there *may* be an error.
To make a long story short, none of the hardware was damaged. I had modified config.sys on my DOS CF card to limit the RAM to 16MB when testing on another system and forgot about it. For some reason, this caused my desktop board to lock up when booting from it. The problem with Windows freezing at boot was just a driver issue that appeared at an inconvenient time. The reason Scandisk reported an issue on my laptop was a conflict with an LBA addressing flag and not a hardware issue. I have no idea why gparted saw the CF card as unfortmatted; this remains a mystery, as I have scanned the drive in Windows ME and Windows 10 and there are no problems with the FAT partition.
What about the floppy disk? That disk ended up being damaged and an unreliable source of testing. Another disk I tested afterwards booted to DOS just fine.
There was no damage from the inadvertent power-on by CPU insertion. Just a comedy of errors that made it appear that way.