I got an IBM 5150 a couple years ago. It was in very good condition and had an IBM 5153 CGA monitor. The system powered on and reported a memory error in bank 0. I can't remember exactly what I did to diagnose it, but I determined which chip it was and actually found a matching chip on another old board I had laying around. This was my first time ever working on the internals of a Pre-Socket 7 computer, so replacing soldered memory chips was completely new territory.

Long story short, my technique, my tools and my soldering knowledge were quite far behind what was required for this project. I ended up tearing a couple of traces off of the motherboard when removing the stubborn RAM chip. I was really really mad at myself for doing this. Thankfully, everything is so large on these old boards that it wasn't too difficult to solder in a new chip and run tiny wires from the two or three legs to where the traces were leading. This fixed the problem and the system has been running like a top for two years now with no problems.

The second stupidest thing I can recall doing happened not long after this, involving the same computer. It came to me with a 20MB Miniscribe 3.5" MFM hard drive. I had to tweak a few things to get the drive working initially and it mostly worked fine after this. At one point I was having a minor issue with it and decided to take a look at the internals, as it probably needed some lubrication (I'd fixed other old drives this way too). All was going well with the drive open when I saw the tiniest bit of dust had landed on a platter. Not a big problem, I thought. I'll just blow it away and if its still floating around in there when the drive is running it'll probably just get lodged in a vent and not be an issue. Too bad I was sleep deprived and didn't notice I had a tiny bit of moisture on my lips. I ended up blowing a few teeny tiny droplets of spit onto the platter. Yeah, I actually SPIT on a working 30 year old hard drive platter. Mortified, I got out a brand new microfiber cleaning cloth, a tiny bit of isopropyl alcohol (after doing some research which indicated that magnetic storage is fairly resilient in this regard). I buffed the platter clean, put it back together and it has worked without any significant problems. Once or twice I had the hard drive make some extra seek noises when trying to play a game that was already on the drive at that time, but it actually stopped doing this after a while and I haven't had it do this almost a year.

Lessons for others to learn from my mistakes:

1. Don't tug on anything attached to a circuit board you intend to keep. If the part isn't coming free easily, get better tools, change your technique (preheat the board, clean thoroughly with alcohol, apply flux, apply more solder, do not burn the board\traces with more heat) or ask someone for help.

2. If you are going to open an old drive to fix it, try to do it in a dust-free environment (not likely) or at least keep compressed air handy... don't spit onto an old hard drive platter. 🙁

My latest screw-up: Not realizing that the CPU died in my Dell Inspiron 8000 laptop, what happened was it froze just before Windows loaded the desktop, I then tried to power it on again only to see it power off with nothing more than the keyboard lights lighting up for a few seconds. I originally thought the RAM needed to be reseated so I tried reseating the RAM sticks and even going as far as trying my spare SO-DIMMs but that didn't help. After a while of brainstorming, I replaced the dead 1 GHz PIII CPU with its original 700 MHz PIII and it finally came back to life! I guess the CPU heatsink wasn't adequate enough to satisfy the CPU upgrade that happened several months ago. 😵

The worst part is that I was frustrated enough to the point where I bought a P&R Dell Latitude C800 (same laptop, just slightly different looking) to use just for parts to fix this one, on the bright side, at least I may be able to refrain from buying individual parts for these laptops the next time something goes wrong.

Was somewhat into DOS retro gaming already in the beginning of the 00's, had kept a 486DX and a P133 machine for the purpose, and of course loads of spares for those.

At some point I decided to clean out my computer part stashes, without really needing the space. Just wanted less stuff. The abovementioned computers, everything ISA and VLB including some nice Tseng Labs gfx cards and a great selection of sound cards, most older PCI cards including a few Voodoo ones, all CPU:s and mobos older than Socket A, all 72- and 30 pin SIMMs, and of course a bunch of noisy old HDD:s and CD drives I don't miss particularly much, all went to the happy hunting ground.

So incredibly stupid...

Haven't really managed to do any screwups while working with my retro rigs though, not even forgotten the mobo standoffs, or made any of the other common mistakes. So far at least.

Many years back my first attempt at trying to build a 486. Had a board that probably didn't work so thought I'd test it out. Stripped out an old 8088 setup and used the case.

I couldn't get it to work, and whilst testing I had the old MFM HDD connected to the same power cable as the CPU fan, with the whole rig precariously balancing on the edge of a chair. You can probably guess what happened next - the heavy HDD took a tumble whilst switched on, wrenching out the CPU fan and pulling the whole case down on top of it.

To make matters worse I managed to blow the PSU by changing the voltage switch on the back, like many here have tried and encountered the same explosive result.

As if things couldn't get any worse, I figured that as the PSU was dead, then the whole case was worthless and chucked it, leaving the old 8088 motherboard behind a drawer where it promptly got scratched and several SIPP sticks broken or lost.

Thankfully I learnt a lot since those early days.

The only other disaster I can recall was trying to use fdisk to erase a floppy. Goodbye HDD partitions. 😲

Not PC related but tried to convert a VCR that had a UK plug to the standard EU one.

I managed to rip the whole power section from the PCB, just a bit after I bought it and brought it home.

i was trying to remove the heatsink on my 430tx motherboard to put a pentium 133 in it and snapped the bottom tab off the socket 7 when trying to get the heatsink off. and my other screwup is i spilled coca cola on my dell at101w mechanical keyboard and most of the alps switches got ruined, and the third one was i ruined a pentium 4 motherboard when i was attempting to recap it, i was a noob at soldering so i attempted to solder new capacitors on it and i completely screwed it up rendering the motherboard useless, one of the caps were backwards and i bridged some of the connections with too much solder.

I did just remember i did broke 19 inches DELL (SONY Trinitron) CRT....
I was moving this monitor to a better location for storage, bcuz i will not need it to lay around and to take space that i need. So i did move the CRT to other room, i did measure the space from the wardrobe to the ceiling to be sure the CRT will fit, i did remove also the CRT stand to make the CRT more space friendly. BUT seems i did not measure 100% and the damn thing did not fit for like what 1-2 centimeters i was like the **** 😵 😵 😕 😒 . So far whatever, but the thing was as this thing is heavy and was above my head to try to fit it on the wardrobe, and the moment i did take off the monitor to bring it back to the floor, i did lose equilibrium and only way not to fall on my face on the damn wooden end of the bed was to drop the CRT in the air. And i did so, and for my luck the monitor did fall on the soft middle of the bed, i was like 🤣 , and then it did happen, the monitor was heavy so he did fall with good amount of force so it did jump back a bit higher on the bed and on the floor and that was about it 😵 😵 😵 😵 😕 😕 😒 😒 😠 .......

Well actually i did not brake the monitor on 100% it was still working, but when you hit the screen even a little bit was losing the signal and recover, so it was not good....
Is never nice to brake an working hardware, or other working thing, but i guess s**t happens........