Adam over on Men In Aprons has a great post about learning from our mistakes. For example, it took only one grease fire in my oven for me to learn to put greasy/fatty stuff on a cookie sheet with a rim instead of a flat cookie sheet.
Except for that, I've been able to avoid any MAJOR screwups. At least everything I have prepared has been edible. And I've poisoned no one.
I have however had a lot of meals that turned out to be, shall we say, less than a raving success. I make lots of notes in the margins of my cookbooks about what went wrong and how to modify the recipe to make it better the next time.
I also never serve anything to guests that I have not already been successful with at least once before. It's like that rule about demos and presentations: Don't demo anything you have not already done before.
I am also one of those Type-A personalities that if something did not go well, I tend to work on it until I am satisfied. Tonight's crab-stuffed mushroom caps were a perfect example. Just not 'crabby' enough. Gotta work on that...