Tell stories about annoying faults in either repair or protyping/etc in electronics!

Way back in the 70's amongst other things I was responsible for designing a switch mode power supply operating from telco exchange battery (-48V) I dont recall there being any control chips available so it was a minimal component self oscillating design, of course no Mosfets so bipolar and around 500W output so quite a big job. Anyway during debug occasionally it would go unstable and as the transformer was hand wound made quite a lot of noise, the frequency would rise along with the output voltage till the output electrolytics blew making a fair old mess of our lab. Returning from lunch one day I found my colleges had built a cardboard wall from floor to ceiling around my workbench complete with a door. I shall never forget everybody diving for cover when a soft whistle was replaced by a rising shriek hahaha BTW it was eventually a successful product.

When I got my Cossor CDU 150 'scope a couple of years ago, 99p on eBay, "spares or repairs" I opened it up before I found a manual for it, pretty soon found the fault to be in the general area of one of a pair of transistors in the channel 1 pre-amp. Tested all the components cold, including that transistor, everything looked perfect. Scratched my head for days until I finally got a manual (£20!) went right back to the same transistor. Checked the signals going in and out of it with an Open University "Generatorscope" someone previously gave me, eventually found that transistor had no gain, it had turned into a pair of diodes. Fine after replacing that tranny, until something else went. On it's 3rd fault now since I've had it...

When I lived on my own in a mouldy smelly basement as a teenage hippy in the '80's, I had an amplifier ('70's era) that had some strange fault or other, can't remember what.
Took it out of it's wooden enclosure, for some reason I had to keep turning it over to test things. Sat on the bed so it could still be connected to speakers & tt. Every time I had to turn it over I pulled the mains plug, which got very tedious after a while, so I stopped doing it. Wasn't long before I was sat holding the earthed chassis with both hands, my thumb on the live mains connection on it's power switch, burning a hole in my thumb (still got a scar that plays up sometimes) and unable to let go because the muscles of both my arms had locked up. I did find the ones between my shoulder blades still worked so I could pull my hands off. Very scary and nearly lethal.
Anyway it turned out to be a thermal fault on a 741 in the end.

More recently, I was given a laser printer that had a very bizarre fault, it would fail to print *sometimes*. Sometimes you'd get 1 colour, or 1 colour missing, or blank pages, or blank half pages, or different colours missing in different places, you name it.
Took that printer apart many times! Eventually found it was transistor on the EHT board presumably had a crack inside it. Obsolete tranny, replaced it with a board pull. Fine ever since.