micmat

Just curious---yesterday I was putting a singlespeed together and got to the handlebar taping stage only to realise the levers were hooked to the wrong wheel
Previous best was mounting threaded headset bearings upside down(surprisingly easy to do !)

commo_soulja

Not threading a new chain fully through a rear derailleur. Quick links aren't so "quick".

cyccommute 

The list is nearly endless. Cross threading the fixed cup on a bottom bracket. Putting in too few bearings in a hub. Putting in too many. Taking a freewheel apart. Forgetting to tighten a pedal (it fell out). Not torquing crank arm bolts. Not checking the brake alignment on my daughter's bike and not realizing it for ~2 years (no wonder she was always so slow) Not tightening the stem on a bike tightly enough. Not tightening the brake levers enough. Not tightening the stem faceplate tight enough. Tightening the bolts on racks too much. Twisting off bolts in anchor cables, brakes, stems, seatposts, etc. Using Phil Wood Tenacious oil for chain lubricant. Using any oil based lubricant for chain lubricant. Using too much chain lubricant. Mushrooming the cantilever brake mounts on a bike so that I couldn't take the brakes off for ~15 years. Pulling out the threads on a crank with a crank puller.

It goes on and on. The only difference between an inexperienced mechanic and a master mechanic is the number of parts they have ruined.

dedhed

That and working for seemingly hours (torch, cheater) trying to get a pedal out and realizing I was going the wrong way

Mr. Beanz

Can't say that I have. I'm known as being very meticulous and as being in the QC field, it's my nature to check my work and verify. I've had up to 11 bikes (only 7 presently) and only do the work on my own. Including building my own wheels, few adjustments for friends here and there. So not as many chances to screw up as a shop mechanic.

Can't say that I've messed up anything up but I did once learn the hardway on a stubborn pedal removal incident, don't push down toward the chainrings.. If the pedal wrench slips, punching the chainrings is a little eye opener.

ka0use

i, too, put the headset bearings in upside down- the ones on the bottom. weirdly enough, they fit better but rode funny.
i pay mucho attention now to orientation.

dcrowell

Too much chainlube is a common mistake of mine. Ugh, messy.

I had a nice hole in my hand from a chain ring incident.

I've left many bolts not-quite-tight-enough and had things rattle apart/slip/whatever.

The biggest problem is probably ignoring when the bike needs maintenance.

Trakhak

Another member of the chainring/pedal wrench/knuckle scar club. Draping a shop cloth over the chainring would have taken only a second.

RaleighSport

not the worst.. but probably the funniest and only happened a few days ago. I was tuning up my main mtb (a franken SS that I put too much time into always) and apparently I forgot to actually close the QR skewer, anyhow I rode it for about 5 feet before I tried to do a bunny hop to see just how much I had lightened it... needless to say my front wheel took off without me, I managed to land on my feet and hold the bike, but it was definitely humbling.

KevinSCo

About a week ago, I was heading downhill at about 35 mph - the first leg of my standard ride, which ends at a "T" intersection with Pacific Coast Highway - when I discovered that I had no front brakes. Forgot to close the front brake tab after fixing a front flat. Was able to stop with the back brake but, man, that intersection was coming up awfully fast!

fishymamba

I was adjusting the stem and took off the headset bolt. Lifted the frame and the fork fell out along with all the headset bearings.

david58

Gotta give blood, though.

At least I've never made it through any maintenance or repair session without doing so.

(I guess I leave my QA/QC brain at work, for I screw up stuff right and left at home. My wife is convinced that since I'm an engineer, I think everything should be easy....).

xenologer

Spent an entire day at a bike kitchen buiding up a full bike from used parts.
Riding home that night hear creaking noise, close inspection in bright lights reveals crack at chainstay/bb weld.
had to restart entire project.

black paint + used + aluminum frame = never gonna do that again

Artkansas 

When I was preparing my bicycle to go on a trip to Europe, apparently I forgot a spacer for the back wheel. I discovered it when I got to Gatwick and was trying to assemble the bike. Kind of made it hard to use the quick release. That's when I noticed that my motorcycle owner's manual was in my back pocket. That's what you get when you have to move and get ready for a trip all at the same time.

Rowan

Cutting the outer brake cable at the top tube stop, with the inner cable still inside.

And just about all of cycccommute's list plus a few more.

cyccommute 

I've experienced that idiosecond as well.

bud16415

The number is countless but the first one that came to mind that I still turn red about is test fitting the seat post in a Schwinn Le Tour I just got done polishing out the top 10 inches of the tube, straight seat post slippery hands and down she went. Two hours later I thought that’s where she’s going to stay.

Wuz

worst thing was in my very younger days..
I upgraded bikes & didn't realise there was more than one size spoke wrench.
There I was rounding off my nipples for a year until I got yelled at @ the LBS.

missjean

Oh, I've done that too!

Ron Harry

Hmm...last week, a brand new bike, had it less than a week...but the cable housing to rear derailer didn't 'fit' perfectly on the down tube support [pooched out just a hair, creating small force on fork]...so thought I'd just 'trim' a bit of the 'cable housing' by slicing around it with a box cutter knife, ha. You know, just a little fix before I'd take it out one morning.

Of course, the cable housing is reinforced with wire mesh and another inner cable. I somehow had in my head it would be like cutting electric wire, where one can cut back the plastic outer wrap. Well, I knew better, but had a mind fart. Anyway, the little one minute fix turned out that I had to install a whole new cable [old one ended up fraying as I had to withdraw it all the way past where I needed to 'cut' the cable housing], so, had to 'restring' the ultegra 6700 sti to install. I'm an idiot...but learn what 'not' to do anyway.

tornado60

Finger tightened rear wheel after repair. Planned to complete in morning before ride. Forgot. Bike let me know after take off. No injuries, so could a been worse.

dnuzzomueller

Every time I loose half a quick link in the back patio bricks and say: "Why didn't I just put down some cardboard!"

steve0257

My excuse is that I was in 5th grade at the time. For some reason I needed/wanted to remove the stem on my bike. This was a quill stem and I started loosening the bolt that holds it in. Took that bolt all the way out. Seemed to take forever to get that little wedge out of the head tube.

sbattey

Dumbest mistake? Not tightening up the shifter levers after fixing my Varsity up and then consequently not understanding why my bike would shift up after a few yards (and then consequently posting here to ask about it, and then figuring it out literally 5 minutes later.....)

Kip

Yep. Been there too.