One of the dumbest things I've done as a dev... I was a front-end/php guy that got hired into a asp.net company. My boss told me to take down a section of the website. I hopped on the server and started looking at files and saved a config file. 2 minutes later I had 3 devs in my office asking what I'd done that took down our entire network.

Thankfully my boss laughed it after they republished the site, but that was my intro into asp.net development.