Story time! I love hearing horror stories about when things go wrong. It's a morbid curiosity. ;)
Here are a couple of mine. Feel free to share yours!
My very first job in IT was for a small newspaper, and I was tasked with a mail merge to send out letters to all of our approximately 50k subscribers. We used an elaborate envelope stuffing machine, and sorted everything in a way to get the biggest postal discounts we could. So I had to query a database, output the results to a spreadsheet, run the mail merge, print the envelopes, print the letters (in the same order as the envelopes), and insert the printed letters and envelopes into the stuffing machine which would then tri-fold the letters, stuff the envelopes, and seal them. Well, I printed the envelopes and letters in a different order, so the letters were all delivered to the wrong people!
Another instance, I worked for a large enterprise. One of my jobs was writing and maintaining a browser extension for the employees to help them use information across multiple internal company web apps. We wanted to know the usage patterns of the extension, so whenever a user visited a page for one of the internal company web apps I would post an event to the corporate metrics server (maintained by another group). I messed up the code that determines if the user is on an internal company web app, so it started recording events for every web page that every user ever visited. The org that managed the metrics service had a hell of a time figuring out the sources of the DDoS attack!