Hot take: a 4 dollar cotter pin taught me more about my shop than any class
Last Tuesday I had a 1987 Schwinn World Sport on the stand for a bottom bracket overhaul and the drive side cotter pin would not budge. I soaked it in Kroil for two hours, tried my punch three times, and finally a guy from the shop two blocks over in Eugene walked over and showed me to back the nut off flush first and hit that instead. Pin popped in 30 seconds and I just stood there feeling dumb about the last six bikes I fought with. What little trick did somebody show you that made you feel silly for not knowing it sooner?