Kind of along the same lines in the sense of being my biggest facepalm moment; many years ago in my first professional programming gig we had full SA access to our sql server. We regularly did troubleshooting via direct SQL statements (a practice I've since shied away from). One particular day, we were troubleshooting a problem in our monthly billing process. I went to delete a few rows and foolishly forgot to include a limiting statement on the WHERE clause, which resulted in most of the data being blown away.

Thankfully, we were able to restore that table from a backup and rerun everything to rebuild the table's state to a correct place ... but suffice to say the experience taught me many things about attention to detail, and how debugging in production should happen.

Good times!