The only big mistake that actually came out fairly good was when I first tried my hand at airbrushing. I was working on a Lightening, attempting to paint it a nice "Lightening Bolt Blue." As with most modelers on their first attempt at airbrushing, I mixed the paint WAY too thin. After about 5 coats, though, it began to take on a battle-damaged, wheathered look that turned out pretty cool. Other than that, most of my mistakes end up in file 13.
A wise man once said, "Learn from the mistakes of others. You don't have enough time in your life to make them all."