It’s common to be asked a question like “what’s the hardest bug you’ve debugged?” at job interviews in tech. This post is about the bug I usually describe. The snag is that it’s quite involved and I don’t actually understand it all the way through – there are one or two aspects to it I often hand-wavingly gloss over. The hope was that by writing it out and fact checking it I’d have a better handle on it; this is what came out.