Making stupid mistakes is a good way to learn. What are your worst?
I once had a high-end mint condition classical guitar that I spent a lot of time trying to sell. Eventually, I found a buyer, and I had arranged to meet him to seal the deal. Just before I left my house, I figured I'd change the strings to give a good impression. I was in a hurry, so in order to save some time, I firmly grabbed all the strings with my left hand and a pair of scissors with my right hand. I figured nothing bad could happen if I secured the strings with my left hand while cutting them as close to the bridge as possible. There was a loud snap, and three nice chunks of finish suddenly disappeared from the bridge area of the top. Of course, the buyer changed his mind because of the damage to the finish.