Ooh boy, confession time.
First one I remember was a Zenix mouse on my dad's 386. I worked for about half an hour to scrape all the accumulated grime off the mouse rollers, only to discover that at least part of what I had removed was a grippy rubber coating. I started to wonder what was taking so long about halfway through cleaning, and confirmed my growing suspicion after I finished when the tracking was worse than when I started. Oops.
Then, my first 486SX had the floppy drives in reverse order from my dad's 386. His 5.25" was A:, mine was B:. I wanted to try OS/2, which needed to boot from 5.25" disks, so I tried to swap the drives. I had no idea what I was doing, and only succeeded in preventing either drive from working. Finally got the 5.25" back online, but the 3.5" never did work again. Still not sure if I actually killed it somehow or just never got the jumpers back on right. Ended up tossing it and buying a new one.
I killed a Cyrix 5x86 motherboard by doing dumb things with the power supply. I had a spare car audio amp and tried to power it with my computer's PSU from a spare Molex. The amp could output more power than the PSU could, and it shut down the PC soooo many times from tripping the OCP. Eventually the VRs near the CPU were so mistreated that I could easily contract heat-shrink tubing with them. That PC was not at all stable.
I've tripped circuit breakers by mixing up the AT power switch leads more than once.
In the 2000s, when it was a legal requirement to have blue lights on everything PC related, I disassembled an Enermax PSU with clear fans so I could add the missing LEDs. That went fairly well until a friend of mine (who was modding something of his as well) bumped the half-assembled PSU while it was powered on, knocking some part of the fan assembly into the PSU and causing it to shut down. That killed the LED and the fan, so in an attempt to salvage the mess, I tacked the fan directly to the 12V rail. It worked for a couple months and then decided it didn't want to be a PSU anymore. I can't blame it.
I recently killed a NOS Teac combo floppy drive. The 3.5" had some issues reading disks, so I tried fiddling with the heads and only made it worse. (I don't think I'll ever bother trying that again. Chances of success are low.) The 5.25" still worked, so I tried removing the 3.5" to replace it with a spare mini drive. That did not go well. The replacement didn't use the same ribbon pinout or something - never worked. So I tried to desolder all the 3.5" stuff so I could just use it as a 5.25" drive, and managed to accidentally melt its connector for the drive head ribbon. At that point, I threw in the towel. I don't deserve to have nice things. 😉
The one that I'm most disappointed by was a recent motherboard disposal. I had one of those VIA Epia 1GHz mini-ITX boards in my Linux router, and it had been running like a champ for yeeeaaars. Eventually the caps dried out and started bulging, and so one day it crashed and failed to reboot. Rather than doing the rational thing and replacing the caps, I figured ... eh... it's done its service... and tossed it out in favor of a replacement Supermicro Atom board. That was about a year ago. I look back now and wonder... what was I thinking??