Mine: not listening to early warning sings that shit won't work. As a result we implemented the wrong technology and had to roll back after a month in production. Lost some customers data, though we had backups, so it was just a delta of a day or so. Still, not pretty, had a rough night writing scripts at 3am to check data validity after.

I will throw in another one from a colleague of mine. He was running some database migrations and ended up pointing the DB migration tool to production by accident. Oops. That one required a restore as well, if I remember correctly.