I once had my Cannondale hanging on the repair stand as I was replacing the three-piece bottom bracket with a Shimano cartridge. I hadn't had access to a repair stand, before I started working at the shop, so I was pretty happy to not be hunched over on the floor, working on the bike.
Because I had not had access to a stand before, I had yet to learn one of the bike mechanic's major rules: Always hang the bike in the stand with the drive-side toward you.
I had randomly hung my bike in the stand with the non-drive side toward me, for some reason, but I saw no problem with that.
I cleaned the threads in the bottom bracket shell, applied a little bit of grease, and started screwing the new cartridge unit in. It was a bit difficult to turn, so i grabbed my wrench and helped it along.
Another important rule: If a part won't thread, easily, from the get-go, take it out and see why, rather than using a tool to overcome the resistance.
I got the bottom bracket threaded in, and picked up the non-drive cup to install it. That's when I realized that I had installed the drive side of the bottom bracket, the side with left-hand thread, into the non-drive side of the bottom bracket shell!
The sharp steel threads on the bottom bracket had nicely tapped out the shell, and the bottom bracket was securely in place...backward. I pulled the bb out of the frame, and was pretty impressed with the Argyle-like pattern that two opposite sets of threads made inside the shell.
I pulled the bottom bracket tap tool down from the hooks, and chased the threads, in the correct configuration, and then attempted to install the bottom bracket, this time in the correct orientation. Luckily, everything went in without a hitch, and the oddly-threaded side of the shell never gave me any problems.
I now consider myself a pretty fair bike mechanic, but I learned a lot of what I know through making stupid mistakes. That's one reason why I always try to do something for the first time on one of my own bikes. The learning curve can be steep, and I don't want someone else's bike to pay the price for my education.