I run a small audio conversion tool. Nothing clever — you paste a link, it pulls the audio, you download it. It handles somewhere around 8,000 jobs a day now, and about 91% of the people who start a conversion actually finish the download. Getting there took five months and four bugs I would not have found in a test environment. This is a write-up of those four. Stack, for context: Flask, Redis, RQ workers, yt-dlp, and a pool of rotating proxies. The setup Requests hit a queue. Worke...