It’s a quiet day at the office, you’re reviewing your colleague’s latest pull request, when all of a sudden a notification pops up. A new Jira ticket greets you: “This stream is stalling on my machine.” But when you play the stream on your end, everything seems to work just fine. Now what? Is it because of an ad break that only happens once an hour? Is there something wrong with their firewall or ISP? Are you even looking at the same stream, or are you hitting an entirely different CDN? We’ve built a tool to record a part of a live HLS stream, and then replay it later on exactly as recorded. Customers can use the tool on their end until they reproduce the issue once, and then send that recording to our engineers to investigate. This way, these intermittent failures that are difficult to reproduce become fully deterministic and reproduce consistently. This makes debugging much easier and faster, since our engineers no longer have to wait until the problem occurs (no more xkcd #303 situations). In this talk, I’ll show how we’ve built the tool and how we’ve used it to debug issues with our customers’ streams. I’ll also show how we’ve integrated it in our end-to-end tests, so we can prevent regressions even for these tougher issues. This talk was presented at Demuxed 2025 in London, a conference by and for engineers working in video. Every year we host a conference with lots of great new talks like this – learn more at https://demuxed.com