The best lessons learned are those leaned the hard way. This one (actually a series of 3 in the one event) is 30 years old and it has never be forgotten.
My wife, not long married, had a work Christmas function without partners so I decided to do some maintenance on a new to me 2nd hand car. I had done some of it immediately after purchase (engine oil and rear diff) but had left the manual transmission. I decided I had some time and our TV didn't work in any case so I drained the manual transmission. One of my mistakes that night, and one I have never repeated to this day, is that I drained the oil before cracking the fill point bolt. When I went to fill it not only did I find that I hadn't cracked the bolt but that the person who last did it (I bought the car 2nd hand) had rounded it. After much swearing and carry on I did get it off and got the tranny filled again but have NEVER again made the mistake of draining a compartment (engine, transmission, differential, transfer case etc.) without cracking the fill point.
2nd mistake that night was not having everything for the job laid out in the garage ready to go. I had left the manual transmission oil in the house. This would not normally present a problem but my internal door from garage to house slammed shut due to a gust of wind. Normally, not a problem but...the door handle lock must have been in the engaged/lock position. As I was the only one home, all other doors were locked. This would not have been a problem but my keys were with my wallet and new manual transmission oil in the (locked) house. Found a bathroom window was not closed but it was one of those small windows and I only just fit in to get into the house to get keys and the oil.
I eventually got everything done and at midnight picked up my wife from her function. I was still in a filthy mood, smelled of oil and told her not to ask any questions.
I have self serviced for more than 30 years now and I have never made those mistakes again. It was a series of unfortunate events but a lesson (or lessons) that was learned. I now lay out all tools, parts and oil out neatly before starting the job. I leave my keys in the centre console of the car and I always remove the fill point bolt before I do anything else.