If you've been in IT long enough, chances are you've done something you really regret. Wipe the boss's hard drive? Delete an important backup? Preventing error should be IT's primary goal, but often we lose sight of this when implementing new and better systems, trying to meet deadlines, or when we're just plain not paying enough attention. What was the biggest screw-up you've made? Get that embarrassing story off your chest in the comments.
Personally, I made a couple pretty big mistakes early in my IT career:
1) At my first IT job outside of college, I was working for a small computer services company. We handled IT for several clients, including a medium sized non-profit organization. The NPO had a third party application running its most important functions - something a local company built on top of Microsoft SQL.
After an upgrade of the software, I was cleaning up old test databases and outdated backups from the server and accidentally deleted the production database - only to find we didn't have a current backup. I called the third party vendor in a panic and luckily they had a backup from before the upgrade, so we only lost about a day's worth of work. Unfortunately for me, the vendor called my boss to let him know what I'd done. I didn't lose my job over it, but I could have.
2) At my next job, while configuring an Exchange server for the first time, I turned on the open SMTP relay option, without understanding what it did, while trying to get something else to work. The next day our Internet connection was overwhelmed by the traffic of spammers using our server as a relay, which kept the entire company from being able to access the Internet or send and receive e-mail. Worse, the IT manager was gone and productivity was crippled for two days while we waited for him to get back and figure out the issue. I didn't lose my job over this one either, but I often feel like I probably should have.
Both were pretty humiliating experiences and I'm embarrassed to admit them publicly - but I'm sure at least a few of you can share some humbling narratives as well.