Very first job, as an intern, I was tasked with building a "free text search engine" for the product, using their api. Maybe my first week or so there, I left a script running over lunch. Turns out the internal IP addresses weren't subject to the rate limiting, and my script's queries were growing exponentially (I was sending the response back to the same endpoint which was querying with the response, and giving me back a larger response etc..) Within 20-30 minutes or so every production machine was stuck running one of my queries. And it happened on the day that the engineering team were taking the new intern out for a team lunch...

At the time I was mortified, but in hindsight the fact that I was able to do that in the first place was really the issue, not my script.