Lacing a new wheel incorrectly and not realising till I was on last few spokes. Luckily before I brought the spokes up to tension.
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Iāve done that too, and got to that point before deciding that the hassle of sorting it was more painful than the annoyance of looking at it! 6 years later itās still annoying me 
..,. And of course thatās not the wheel Iāve crashed, or broken spokes on, or the one that cracked at various spoke holes - itās been faultless!
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You should be arrested.
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They would have caught me too as I was stuck on the side of the road with a dropped chain!
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I just did this 2 weeks ago 
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I have the same setup. Whenever I do things for the first time, on my bike, I examine things ad nauseum. I looked over my new rings and noticed that the inside is machined with a little seat for the torx bolt. Otherwise, itās hard to notice.
My worst mistake was pushing back the Ultegra hydraulic brake pistons with a metal tool - - which I should throw away. Cracked, I totaled the calipers on the very first time I changed the pads.
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I canāt tell you how many times Iāve put a new chain on in a rush and accidentally looped the chain through the outside of the derailleur cageā¦.it actually shifts fine that way btw. Itās just a little louder 
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I did this on Monday. 
What is the point of those tabs anyway? How is the RD working now? Makes me think of just cutting it offā¦
Yeah I donāt think Iām missing anything without the tab 
Think itās just BIG BIKE keeping mechanics in a job
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You can buy aftermarket pistons, they are arguably better than stock. Last set I bought were Ā£20 which is cheaper than any caliper.
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Not a home mechanic mistake butā¦ I once picked up my bike from the shop and it felt disturbingly off-kilter. It turned out the cranks were one notch offset from 180Āŗ!
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I was setting up some road wheels as tubeless a couple weeks ago. I was using the Reserve Fillmore valves for the first time. They donāt have removable valve cores so you just pour/squirt the sealant in rather than taking the valves out to fill the tire.
I filled up my tires to 80 psi to get them seated then attached a funnel to the valve to pour some sealant in. But it wouldnāt go in. I stood there semi-perplexed why it wasnāt going through the valves into the tires. Shook the the funnels a little bit to try and get it going. Still no luck. So I decided to poke a hex key through the sealant into the valve. Suddenly sealant goes shooting out of the funnel everywhere. Whelp, turns out my mistake was I forgot to deflate the tires before adding sealant.
Wont make that mistake again (probably)
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Last night in a hurry I put a different front wheel on my MTB and not paying attention I put the wheel on backwards (yes, that means the rotor was not on the right side).
By pure luck when I put it in the fork the brake caliper sat right between (2) spokes and because I was not paying attention I just spun the wheel to hear some horrific sounds of the spokes blasting the side of the caliper and taking all the paint off.
Poor XTR caliperā¦.
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Most recently, was swapping to a medium cage derailleur to accommodate a bigger cassette, which would mean a longer chain. Sized and cut the new chain using the old cassetteā¦
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I have opened the bleed port on my sram axs shifter while attempting to replace the coin batteryā¦ twice 
Both times I was cursing sram for making it so difficult to replace a batteryā¦ until I realised what I had done, then the cursing was redirected at myself.
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First one that springs to mind for me is the first time I ever tried to bleed my own brakes.
At the time $30 a brake or whatever it was seemed too steep and I liked working on my own bikes so why not. Two sets of contaminated pads and rotors later I was out closer to $200 I think?
Of course when I got the bike back from the shop on my first ride out the pads fell out because they forgot to put the fixing pin back inā¦soā¦
More so though my issue is just trying to be kind of cheap here and there and that resulting in way more money spent in the long run. Most recent was this last winter I got my first road bike and thought Iād go all in setting it up fully internal hose routing. But it turns out Iām still the cheap guy who didnāt want to pay for a brake bleed so after well over $100 in mismatched Aliexpress parts, a full Ritchey setup which still didnāt fit, I finally caved a bought the FSA stuff I shouldāve bought from the jump. Of course even that required a wonky 3D printed part for my bike so it doesnāt look quite right.
If I had just gone for the Bontrager stuff that would be made for my bike from the jump probably wouldāve saved a couple hundred bucks 
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Oooof I have learned this the hard way about many things!!
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Arguable if this one is actually my mistake, but: Brand new S Works Epic 8 Frame (2024 Stock.) Apparently Specialized lists different torque values in two different places for the Shock Extension, which I didnāt realize until Iād stripped the threads in a $230 carbon part. Iām not convinced it wasnāt torqued wrong on initial assembly either, as it broke real, real hard when I was taking off the shock.
With my bike out of condition - quickest resolution was just order the damn part instead of going to a dealer to argue about a warranty replacement.
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Youād think I wouldāve learned by now too. But thereās just so much shiny cheap bike stuff out there! Admittedly the tariff situation in the US has slowed me down significantly lol
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We had a friend who got a brand new Rudy gravel fork on a killer deal for his new build. He measured the headset and spacers he would need to get the proper stack height and cut the steererābut forgot to include the height of the stem 
. Itās slammed with an upright stem on it nowā¦
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