rsbob 

This morning I removed the old Garmin bar mount for one that attaches to the stem. Had the mount positioned correctly was alternately torquing the bolts when the left one (6 nm) just started slipping and wouldn’t grab. Oh crap! No purchase what so ever, and it wasn’t cross threaded. Thought today’s ride was doomed.

Removed the bolt and didn’t look bad, but tossed it anyway. Had a spare and used that, and no problemo it went in and torqued right down. Didn’t want to have to buy a new stem or rethread it and put in one over size - what a pain.

Then there was the barrel adjuster on the rear mech that I over tightened using a pair of pliers that now only spins and does nothing, unless I use said pliers to grab and squeeze it. Don’t know if Shimano even sells a barrel adjuster assembly. Probably should take a look - and stay away from pliers in the future.

ScottCommutes

Besides every time I lose a ball bearing?

Jughed

Hand dcrewed the bolt that holds my disc brake into the fork - went in smooth, no issues. First turn of the torque wrench and the threaded adapter inside of the fork just started spinning.

genejockey 

Putting the stem and bars on, then the levers, measuring carefully to get them in the right place. Carefully cutting the housing to just the right length, an running the cables. Then discovering you installed the stem too low, and you need to raise the bars about an inch and angle them up slightly so that my perfectly sized housing is now somewhat strained.

Spending 5 minutes working a beautiful Cinelli 1A stem onto a pair of Highway One bars, only to discover it won't clamp because the bars are 26.0 and the stem is 26.4, which I knew when I bought it and subsequently forgot..

Spending 20 minutes working a beautiful SR stem onto a pair of Highway One bars, only to discover they won't go onto the swelled center of the bar, because the bar is 26.0 and the stem is 25.4, which I knew when I bought it and subsequently forgot.

Carefully removing some lightly-used brake cables after dabbing some glue on the cut end so that you can reinstall them when you put everything back together, and then when you go to put everything back together discovering that there's a blob of the glue on the end of the cable that makes it too big to go back into the cable housing.

Pulling the above blob of glue off the cut end of the cable and trying to put it back through the housing, and ONE STRAND of the cable gets caught on the housing and bends so you can never, ever get it to lie flat and fit down the housing again.

Getting a flat on a ride, pulling the wheel, working the bead off the rim, pulling the old tube and carefully replacing it with the spare in the saddlebag only to discover the valve is too short for the pump head to grip it well enough to pump the tire up. Pulling that back out, getting the old tube and finding a "snake bite", insta-patching that, putting the tube back in and finding the tire won't hold pressure. Then repeating that two more times.

That last wasn't so much an "Oh, crap!" moment as it was a series of "Aww, ****!" moments.

Yan 

One time I hacksawed a steel steerer with the star nut inside. Sawed straight through the star nut and had to hammer in a new one.

delbiker1 

Too many, too often, some that I don’t care to share. I will say that the ones that get repeated are the most frustrating, and bring out the self flogging.

pdlamb

Some years back, I was tasked with collecting "lessons learned" (an institutional requirement). One co-worker told me, "I never say something was a lesson learned. It may have been a lesson available for learning, but I've never seen this organization learn a lesson."

Aardwolf

Trying a new BB for size on a newly painted frame.
Decided to tighten the crank bolt before checking the clearance to get an accurate measurement.
Through the paint on the chain stay down to the metal.

BB works perfectly with a 3mm spacer: 2 mm gap instead of -1mm.
Chain stay paint fixed 3 months later..

genejockey 

Horse....water...drink.

CAT7RDR

A couple times I have made changing a flat into a lesson of futility.
First time, double flatted when I could not locate the cause of the puncture. Finally, went over the rear tire painstakingly to find a sliver of steel embedded in the tread only visible when flexing the tire.

Second time, bent two Presta valves with a faulty minipump (likely user error).
A roadie graciously gave me a tube and a CO2 cartridge to get back to my car.

veganbikes

Usually it is poor routing of a chain through a rear derailleur I will sometimes just do it mindlessly and then realize oops wrong way. It doesn't happen super often but every once and while.

Mtracer

Broke an aluminum chain ring bolt because I didn't appreciate the difference between greased and dry torque specs. Fortunately I was able to "borrow" one off a crankset I wasn't using in order to make the next ride.

CrimsonEclipse

Mine was:
Wait a second... when did that 1/4" gap in the frame tube happen?

RCMoeur 

Finding the "perfect" crank on a used frame. Pull the bolts, start the puller, feel significant resistance, stop, apply penetrant, wait. Try again... and watch the puller "pop" out, taking the little aluminum shavings of the former threads with it.

And many years ago getting in a hurry installing a bottom bracket, and realizing that I cross-threaded the fixed cup. Put it back in straight with copious amounts of blue Loctite, and it seemed to hold. At least for a while.

RCMoeur 

*cough* JB Weld *cough*

CrimsonEclipse

Sir, I am a Professional.
Drill some holes, baseball stitch it up with .064 safety wire.
THEN JB weld.

"Beat it to fit, Paint it to match."

RCMoeur 

Test fit crank. Looks like lots of chainstay clearance. Tighten it down - and where did that chainstay clearance go?

RCMoeur 

Er, um, that's what I meant. Fixed several broken racks that way. And they've held up OK. Another application of those reinforced concrete classes I took in college.

MarcusT

They do not get lost. They know exactly where they are going. They bounce deliberately and end up in a nook or cranny where they know they will stay well hidden. A few even made it to the floor drain 12 ft away
Oh, and their friends, the tiny screws as well

rsbob 

Epilogue to the new computer mount: Was about 25 mile into today’s ride and the bars slipped down while braking and hitting a bump. Got out the multi-tool and snugged the top two bolts, but did not use a ton of force because didn’t want to over-tighten and strip another bolt and be screwed so many miles from home. After another 10, rolled into a bike shop and asked if I could borrow their torque wrench. They said, they would prefer to do it and took one look at the bolts and said the top and bottom bolts were not equidistant and they were also at a slight angle - so they removed the computer mount and said it was a Scott mount and would never mount up correctly to my stem. I told them to s-can it and to fix my poor job. I insisted on paying for their time even though they said, “no charge”.

So now I am out $15 for a bad mount
$27 for the repair
1 stripped bolt

And now the old Garmin mount will be reinstated like before. Luckily all of it together was cheap and I didn’t crash when my bars slipped. Doh!

Doc_Wui

-Used a crank arm puller and forgot to remove the bolt that held the arm.
-Mounting tires with the rotation arrow backwards.
-Cutting a threadless steerer tube a wee bit too short.
-Mounting an inner tube with a double twist in it, so it's like a balloon dog inside the tire.Hey it was OK til I had a punctture a year or too later,
-Cutting a little metal off a seatpost tube to allow a lower seat and finding that the remaining stub was too thick for a quick release clamp. Had to use a boot on clamp.

Smaug1

I did The Classic noobie mistake a few weeks ago.
I had my new bike up on the work stand. Wanted to change the stem. Anyone with experience knows you don't do that on a work stand.
I took off the stem cap and the stem and all the head tube innards and spacer rings and washers from the top dropped out all over the floor. I didn't find one spacer ring and had to buy another one.

Lesson learned.

Wileyrat

I almost did the same thing yesterday. I was moving my spacers on my mtn bike to raise the bars a bit, and as I was loosening the screws on my stem, I realized what was about to happen. Fortunately I averted the disaster.