Things like when to cancel early, how to handle par2, obfuscation, rars within rars, decoding articles. Just a mountain of "huh, didn't think of that being a complex problem" that was resolved over time.

I was able to replace the arr suite + sabnzbd with my own rust+ts frankenstein but had I known how complex the usenet piece was going to be I would have separated it into two phases.

sabnzbd is basically packaged decades of domain specific knowledge about deobfuscating media files