Not the worst, but certainly most infamous thing I've done: I was testing a condition in a frontend template which, if met, left a <!-- leo loves you --> comment in the header HTML of all the sites we served. Unfortunately the condition was always met and I pushed the change without thinking. This was back in the day when bandwidth was precious and extraneous HTML was seriously frowned upon. We didn't realize it was in production for a week, at which point several engineers actually decided to leave it in as a joke. Then someone higher up found out and browbeat me into removing it, citing bandwidth and disk space costs.

Now, if you go to a CNET site and view source, there's a <!-- Chewie loves you --> comment. I like to think of that as an homage to my original fuckup.