The biggest mechanical mistake I made was the day I drove my Ferrari too fast around a curve and hit a curb. The damage cost me nearly $6000.

Disassembling a hub without thinking and watching all the little steel balls disappearing on the garage floor/lawn/drain, done it several times even......

A) Forgetting to tighten the handlebar clamp on a bike that was about to be test ridden by a customer.

B) Letting a screwdriver stray into the spokes of a spinning rear wheel, launching it across the room, almost hitting my boss.

slapping a chain on a cheapo mtb, not loosening it up at all, not noticing that the link isn't sitting quite straight, having it catch in the front derailleur, bending it and breaking the chain. walking home. grrr.

I built a fixed gear for fun, and hadn't installed my front brake, but decided to take it for a 5 mile spin. A block from my house I was going down a hill and decided to try my darndest to skid, at which point my cog/lockring flew off and I was barreling down the hill with no idea how to stop. A friend had told me once you can fake a brake by lodging your foot between the frame and tire. I didn't want to take my eyes off the road since I was bout to pass through an intersection, and so I pushed my shoe against the front wheel. My foot slipped to the side and my toe went through the spokes.

I promptly flew face first into the pavement, blacked out, crawled off the road, and called my roommate to take me to the hospital. I suffered a concussion, a broken clavicle, and sprained all 5 fingers in my right hand (not to mention some road rash).

Lesson learned: If you want to try building things that can fly down hills, make sure you know what you're doing! >:D

installed my drive side, on the wrong side

When I routed all of my cables on my road bike without ever riding it to find out if the bar placement was comfy. It wasn't, and I had also cut all the cables too short, so I couldn't adjust the bars up (which I needed to). So I had an extremely expensive, and extremely uncomfortable road bike.

I have since recut and rerouted all the cables, and now it feels great!

I have two, first one was I tried to use duct tape as a shim to shim from 31.8->25.4 for my handlebars. I rode home ~ 5 miles grasping the stem, going about 2 mph, probably would have been better off walking.
Second was riding with loose cranks. walked home ~3 miles that time after failing to tighten them with a house key.

Mistakes:

Welding with a hole in my glove...
Forgetting to tighten the stem on my tallbike.
Pinning hand between crank arm and chainstay while spinning/truing wheel.
Don't think I've ever crashed or really ruined anything due to my wrenching... I'm usually pretty good as long as I am not completely tore up.

I let a bad rim strip get the best of me about 3 times. Then reused it when I really needed a rim strip, and didn't want to go to the shop. Got screwed over again on a tube.

I did the exact same thing...except it hit my little and ring fingers. No stitches (I should have gone to the hospital, but I just patched myself up). The best part? my hands were a little greasy at the time and I couldn't get it all off without effing up my knuckles even more, so now I have a small 'tattoo' from the grease in the cut where my fingers hit the chainring.

I ordered 26mm bullhorns for my 22.2mm quill stem...oops.

I had a creaking sound, disassembled the bb, cranks, reassembled them, creak still there, but it's only audible when riding. Turns out it was the rear rack. I put a piece of felt between the rack and the frame where the creak originated... problem solved.

Lol, these are all awesome. My 2nd one:

I had my test-build fixed gear upside down in my living room and I was pushing the crank arms around with my finger to see if the chain was balanced. I was focusing too hard on the chain and stopped watching where my finger was. On the last pass my finger actually went into the chain's domain and my finger went halfway up the drive train with a chainring tooth going right through my finger nail. I couldn't just pull my finger "out" of it (it was totally stuck), so I had to actually backpedal a whole rotation to "eject" my finger.

Needless to say it hurt.

This made me a bit nauseous when I read it.

I'm fretting the day I do this once my FG is built.

For stupid mechanicals, I've had a few MTB tubes pop on me after not being seated right (concrete basement + blown tire = loud)and I've cross-threaded my aluminum MTB bottom bracket, but no horror stories yet.

forgetting to tighten the bolt on my rear wheel.. and then riding 2 miles while carrying 10lbs of cat food. No accidents tho, thank god.

maybe more to come when i build up a bike.

Not mine, but I've fixed countless friends' QR levers. Apparently they think they're safe to ride if you just tighten a bit without flipping the lever. I'm sure I've saved them from some pain down the road, if not their lives.

Not really mechanical, just me being dumb.

Biked to the liquor store... nowhere to lock up, detached brake and popped front QR on one of my old conversions as a measure of security. Buddy and I filled backpacks full of beer and took off, racing the 1/2 mile back to his house. Got to his house before I realized my brake wasn't attached and QR wasn't engaged.

Cut the steerer too short on a rigid setup.

$100 and a brand new fork down the drain.

Forgot to tighten my bottom bracket lockring one last time and watched in horror as it slowly came loose. I also managed to strip my spare wheel, taking threads off of it, then promptly destroyed the threading that cog on my main wheel, smashing threads down. I wrecked two hubs in a half hour. I was really pissed.

Thought it was a good idea to clean the chain on my (then) motorcycle by putting it up on the center stand, engaging first gear and grabbing the chain with a rag. Rag got caught in the rear sprocket, as did my thumb.
Lost just the tiny tip of it, but there was blood all over the place.

First time I bought a track frame from a person, than a store.

Came with frame, fork, headset, bottom bracket, and cranks.
Took it out and thought nothing of it.
Was jamming pretty fast, then the drive side crank arm popped right out. Had to Ted Shred into a parking lot.
Didnt check if there were any crank bolts, when I got the frame.

Ruined a new Izumi chain.

trying to talk sense to bike forums

Trying to fix the shifting on my mtb, I ripped all the threads out of my derailleur's barrel adjuster. Got a fixed gear bike with in a month I was so pissed.