this, like all other bicycle maintenance, is a skill that has to be learned. When I first started working on bikes, I made TONS of mistakes (under-tightening and over-tightening my stem faceplate being two of them). I remember mashing up a hill and having my bars roll under on me. Scary.

I've also had crank arms fall off the bottom bracket while riding. And hubs stip out. And pedals detach from the pedal spindle. And had clipless systems fail. And seatposts collapse into the seat tube. And seats rotate upwards on me because I didn't tighten down the clamp enough. I think that's probably most of it...

Now, though, I consider myself to be one of the better wrenches I know. Certainly better than the guys who work at most shops around here.

Just don't be such a puss about stuff and learn from your mistakes.