Ask HN: What was your most challenging technical achievement?

Inspired by the following comment on HN:

> I once had to spend 6 months reverse engineering a video output ... it turned out to be a proprietary video signal invented in the 80s that had long since been abandoned by almost everyone so I then had to build a fucking circuit to decode it

I'm sure there must be many others on HN who've gone through something like this. Would be awesome to read your stories :)