Mistake #1: Don't Rush Your Work
This was a tip I had a little lower, but I decided to move it up to the top. This might be the number one way to increase the quality of your wrenching. It would help if you worked on your bike only when you aren't rushed and have the time to allocate to do it right. It doesn't matter how many tips and tricks you follow, and if you don't pay attention to details and you don't take your time, the job will not come out correct. If this rule isn't followed, you're going to have a bad time.
Mistake #2: Incorrect clothing
We're not saying that donning a mechanic shirt is going to make your stem bolts torque to a more specific value, but we are saying that if you're wearing a light shirt or a pair of shorts that you don't want to grease up. Wear something dark and as non-porous as possible. Think Dickies shorts and a dark shirt.
Mistake #3: Incorrect or Blunt Cable Cutters
This is one of those mistakes I see all the time. Using an old pair of cable cutters or maybe even the wrong type of cable cutters for the job is going to leave your cables frayed. Frayed cables don't last as long, and you'll end up spending more in the long run.
Mistake #4: Rounding off bolts
This is one I'm particularly guilty of. It's also something we could speak to at length regarding the causes. But at the end of the day, the tool you're using is probably made of very hard steel, and an aluminum bolt is usually really soft. So if you use a metric hex wrench in a standard bolt, you're just going to have a bad time.
If you do end up rounding off a bolt, check out "Speed Out" at your local hardware store. It's basically a reverse threaded screw that mounts into a drill chuck and will bite your bolt and twister her out of the ole bolt hole.
Mistake #5: Not greasing things that require grease!
In order to get this one right, you're going to need to consult your user manual. In most cases, two metal services or anything getting clamped is going to require a tiny bit of grease. The reason being is that your sweat can contain some corrosive salts or, in particularly grimy or rainy conditions, that dirt can form an abrasive paste. In the past, I've used special carbon grease for my Seatpost, I've greased water bottle cages, greased by Speedplay pedal bearings, as well as my headset (mostly due to sweat).
Make sure you apply grease as the manufacturer recommends to your items.
Mistake #6: Using the wrong brake pads
This is a pretty easy one to avoid, but many novices don't know the difference. If you frequently swap out carbon wheels, it's best to swap out the pads as well. Reason being, is that carbon wheels sometimes don't dissipate heat as well, and on a long descent, the result of overheating a set of wheels can be catastrophic. Additionally, if you're running regular aluminum wheels, the pads can pick up tiny metal particles over time and grind those right into your carbon braking surfaces.
Mistake #7: Incorrect Footwear
We've all done it. We've wrenched in the old slippers or sandals or maybe without shoes at all. But bicycle tools are often made out of this new hard thing called metal. To make matters worse, tools are typically made from Tool Steel, which is a particular variety of metals that make dropping a wrench on your tootsie all that much more painful.
Mistake #8: Not Respecting your Workspace
This one kind of goes back to #1. If you don't rerack your tools, if you don't have adequate conditions or workspace, good results will be much harder to come by.
Mistake #9: Forgetting how to put back together something you took apart
Luckily, many of the items we take off the bike have fairly straight forward or obvious installation methods. Take installing a cockpit, for example. It doesn't take a rocket scientist to know that you put the levers on before the tape. But, some still struggle. This isn't something I understand, and ingratiating myself with individuals that do such things is not an endeavor I wish to take on, but I digress. The best solution to this common mistake is right in your pocket or potentially in your hand right now. It's your cell phone. Take photos, take videos, make notes on how things were disassembled, and future homer will thank you.
Mistake #10: Overtightening spokes
I've seen this particular mistake a handful of times. Overtightening a spoke can make it more susceptible to breaking. And to be honest, the benefits you get can sometimes be negligible. A tight spoke will make your wheels feel snappy and responsive, but sometimes, going with J bend instead of straight pull and keeping a nice consistent tension will make for a very pleasant ride on our city's crumbling infrastructure.
Mistake #11: Overlubing your chain
Yes, this is a thing, and yes, it can have some negative consequences. The thing about chain lube is you want to use as little as is necessary. I word that very specifically because "what is necessary" can be up to debate. But in my experience, it's a tiny dab on each roller and then wipe off the excess. The problem with over lubing is that the lube can attract and hold dirt, turning it into an abrasive paste that will prematurely wear out rollers. Worse yet, the lube can get on critical components like the contact patch of your tire or on disc brakes or braking surfaces.
Mistake #12: Not Marking off the Seatpost Before Reinstalling
I must admit, this is quite a newb mistake. If you're disassembling your bike, or cleaning/re-lubing the seat post, forgetting to mark off the height is a big pain in the rear. It takes two seconds, take a little piece of tape and mark off exactly the first millimeter of exposed seat post above the frame. It'll save you loads of headaches down the road.