Everything I know about wrenching (not much) I've learned by trial and error on my 2006 KLR 650. I've had it since 2013, and there have been some moments. The worst was during a time I was commuting frequently on it, and decided to get new tires put on. I pulled the wheels to save money at the shop, got the tires on and remounted. A few days (and probably over a hundred miles) later I was leaving work when I noticed the castellated nut on the rear axle was STRAIGHT UP GONE. I don't know if I had messed up putting the cotter pin in, or never put the nut back at all (never found it). I caught a ride home and ordered a new one, bike sat at work a few days - no worse for the error. I shudder to think how badly that could have ended though. Lessons learned; 1) finish the job while you're working on it so as to not lose your place. And 2) thorough inspections when finished. What's your dumbest wrenching (or riding or whatever) mistake with a bike? How did it turn out?