Realised I had run my chain incorrectly around my rear derailleur on my new bike. I had successfully ridden for a good few weeks, until the chain started slipping, and then realised it had been rubbing on a little bit of metal inbetween the derailleur cogs. Somehow when I re-routed the chain and turned it inside out the chain has stopped slipping so I consider my self a bit lucky this time.
^This, took me a while (far longer than it should have) to notice, and the derailleur now bears 2 grooves on the outside of the flange.
Cheapskate fuck-wittery: re-laced my roadie / commuter wheels and forgot to get rim tape. Rather than do the right thing and go get some, I think 'hey, sticky tape will cover those holes'. Run some electrical tape around the rim a few times, thinking I'll change it soon when I get some proper rim tape. That, of course, doesn't happen. I get a flat a few months later on the way to work, so spend my lunch running around getting a few more inners to get me home. Put the first one in, pump it up and get to around 80psi and -boom-, there it goes. To continue my complete lack of thinking, I just think I must have had it pinched & try again with a new tube, same result. By this point I am out of tubes and finally notice the hole in my makeshift rim-tape where the tube has pushed it through a spoke hole. Idiot.
Riding to tennis with the tennis racket off the handlebar by the bag strap. It swings nicely into the front wheel and jams it up, sending me endo onto the pavement.
Converting to tubeless but (once again) being a cheapskate and not purchasing proper tubeless ready tires, instead opting to re-use some Nevegal's I had lying around. Second trail I take a leap, land & tear a gaping hole in the tire and ripping it off the rim, sending me straight into a tree at high speed. Result: Badly dislocated finger & 3 months occupational therapy to get it moving again.
Not checking my bike before a ride, only at the start of the next ride checking why the rear end felt sloppy and noticing a bolt missing out of the suspension assembly. Thankfully after disassembling and rebuilding the rear end the bolt holes lined up again and it all went back together.
I could go on, but I think I'll leave it there for now.