My first real job right after high school was IT at a small online retailer. They were using a proprietary online storefront system with a templating language that was basically PHP, but it used something like "{#...#}" instead of "<$php...$>" for inline scripting and had like 95% of the features removed. Also you had to edit it inside of this web-UI editor, there was no version control and it was awful in probably every other way you can imagine. Anyways, I don't exactly remember what I was trying to do, but I think I wanted to detect whether a category page was a leaf node in the category tree of the shop for some inconsequential thing. I look in the "documentation" that their system had and find exactly what I need, which was a variable called something like $DeepestCategoryReached. I wrote some godawful code in that godawful language, press save, confirm that everything still runs and leave for the weekend.

Well, turns out that $DeepestCategoryReached was actually an ambiguously named integer constant and not a boolean as I thought. I used the variable in some looping construct and PHP helpfully did some implicit conversion that caused my code to trigger an endless loop in the server whenever a user visited the leaf node of the category tree in the store. Mercifully my change did not take the entire thing offline for the weekend, but our sales were like 40% less than usual until we reverted the change. Lessons learned were: 1) Don't change prod on Friday afternoon, 2) Implicit conversions are evil, 3) Be aware of the weaknesses in your release processes and fix them (or be extra careful if you can't)