I think everybody has done this at some point, and I'm sure I will not be the last person to have done it; leaving the WHERE clause from DELETE and UPDATE statements when writing SQL, I caused about 45 minutes downtime on our RDS instance the last time I did it, but since we had multi-AZ setup, no data was lost. I also frequently get mixed up between development and production environments.

Every database alias I have now has the MySQL --i-am-a-dummy flag appended. This has been a career-saver in my eyes.