I don't think there is any point in me mentioning Joel Spolsky's essays here as if everyone doesn't already know about them. I just wanted to quote this bit about your mama from his new piece on the importance of design:

My friend, you can put wheels on your mama but that doesn't make her a bus, and if you think you can refactor your wrongly-implemented file-copy function to make it preemptive rather than threaded as quickly as I could write that sentence, you're in deep denial.