This is nowhere near the worst on a technical level, but it was my first big fuck up. Some 12+ years ago, I was pretty junior at a very big company that you've all heard of. We had a feature coming out that I had entirely developed almost by myself, from conception to prototype to production, and it was getting coverage in some relatively well-known trade magazine or blog or something (I don't remember) that was coming out the next Monday. But that week, I introduced a bug in the data pipeline code such that, while I don't remember the details, instead of adding the day's data, it removed some small amount of data. No one noticed that the feature was losing all its data all week because it still worked (mostly) fine, but by Monday, when the article came out, it looked like it would work, but when you pressed the thing, nothing happened. It was thankfully pretty easy to fix but I went from being congratulated to yelled at so fast.