To my ever-lasting shame: The first gun I bought for myself was an Ishapore Enfield. (This is not the shameful part. Quite the contrary.) Not exactly the most complicated machine invented by man. (I think only the Mosin-Nagant rifles have fewer moving parts...) A few months after buying it, decided to do a full strip and clean-up/restoration, and install a roller-trigger. Got it completely torn down but, rather uncharacteristically of an aircraft mechanic, didn't really pay much attention to how it was coming apart. Put all the bits into cans of solvent to soak off the cosmoline. Set the wood parts in the Arizona sun for a week to heat out the grease...
Two weeks later, pulled out the yellow-book manual to start re-assembly... and literally could not figure out what the heck I was doing. (No, alcohol not involved.) Put everything back in bags and cans and shoved into a cornor... and got busy as heck for the next year, never getting back to it. Finally got tired of tripping over it and took it to a local gunsmith. Set the box-o-parts on the counter and before I could launch into a long-winded explaination/excuse, he simply held up a hand and asked "What make/model is it?" I told him, and said "I guess this happens a lot, eh?"
"Kid..." (I was 40 at the time, so...) "...this probably makes up half my business."
So far I haven't had to repeat the experience. But apparently I'm still young and have lots of time left to make mistakes.