Ask HN: What's the most challenging thing you've worked on?

I've been getting this question a lot lately in interviews, and am having trouble coming up with something. Either it's imposter syndrome and I think that nothing I've worked on was challenging, or I really haven't.

I'm wondering what other people would answer for this question, and how much detail would you give?

I'm specifically asking this question in a Software Engineering/Programmer context, not that interested in manager type challenges.