So as not to embarrass anyone, I'll start first.

After a night of drinking wine by the fire and making well laid plans, I felt really smart and inspired, and bought a domain name and hosting account. Only, I made a typo in the domain name. For a minute this was proof of our success and our bright futures, but then the next morning I realized why nobody had taken that domain!

Your turn. The goal here isn't to make us laugh, it's to point out things we should try not to do. What's the best web mistake you've learned from?