I wrote an update script for a database table not realizing I had the key wrong (I'm kind of fuzzy on the details, but essentially I think it was a composite key but I was only using one of the columns in my WHERE clause...) and accidentally updated all customers addresses in our database to the addresses of one account.

Luckily we had backups from that morning so we only lost any address updates people would have done that day, but it made for some interesting customer service calls for awhile...