There are three monumentally stupid things in bike repair I can think of right off. I know, I've done all of them. The most common is forgetting (or not knowing) that the left pedal is left-hand threaded. This usually doesn't cause dire consequences -- just a large amount of embarrassment. The second is trying to pull a crank arm without removing the washer. Basically, your fine crank puller tool, instead of pushing against the spindle thereby moving the crankarm is pushing against the washer which is firmly planted in the crankarm. Effectively you're trying to move the crankarm by pushing on the crankarm. Sort of like isometric exercise, though not as productive. If you're lucky, you'll discover your mistake before doing too much damage to the crankarm and your tool. If you have a large wrench on the other end, you'll probably ruin both. The third booboo, and my most recent, is disassembling a wheel without taking the freewheel off first. Well, big deal, you say, just put it back together again. Not if you disassembled it by cutting the spokes. You're left with a hub/freewheel combination that can only be separated by herculean effort.
The reason I got into this sad state was that I decided it was time to refurbish my touring bike. Last time I went touring, I got a total of six broken spokes (three at a time, and yes, it is possible to ride a fully loaded touring bike with three broken spokes on the rear wheel). I dusted off the bike and looked at the rear wheel. True, it had six brand new spokes, but I figured that they couldn't be depended upon to do the job should the others crap out, which I had reason to believe they would, since they appeared to be composed primarily of rust and dirt.
Now, I had sworn off wheelbuilding some time ago. I own a fine Park wheel truing stand, all sorts of cool tools, like a nipple driver (no snickering), and about 100 spokes of assorted sizes from the old days when I was into this sort of thing. The reason I got out of this sort of thing was that after spending lots of time building various dodgy ovoid wheels, I realized that this wasn't all that much fun and that I could buy a wheel already built and true for not much more than the constituent parts, and a lot less if you valued my labor for anything (which, if you built wheels the way I do, you wouldn't).
But here I was with a wheel in dire need of rebuilding, a lot of spokes in a drawer that would fit, and I had just gotten all fired up after reading a Bicycling article on wheelbuilding. It sounded really easy when they did it. So I took the wheel off the bike, tire/tube off the wheel. I decided to leave one spoke in, the one next to the valve hole, so that I would know how to start it, always a major quandary. So I took off the six good spokes and cut the 29 others. Which, incidentally were so easy to cut that I'm sure they wouldn't have lasted much longer. It was then I realized that the freewheel was still on he hub and there was no way to get it off. I made some overtures with a chain whip, freewheel remover, and large breaker bar, until I realized that this would probably end up with me gashing my hands on the freewheel teeth and having to be transported to the emergency room bleeding like a stuck pig.
So I rationalized that I really didn't want the freewheel or hub anyway. In fact the freewheel was a SIX speed (can you imagine), and the hub was not exactly as smooth as a Swiss watch. Fortunately, I was able to go to the bike graveyard and find another wheel.
Still the experience has strengthened my resolve not to undertake anything more complicated than changing a lightbulb in the future.
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