Hello Quaid, long time!
There is a type of post that has started to appear in the last couple months that have played havok with Newsbin's ability to unRAR (or even use PARs).
The files I've had problems with have been video files.
The main NZB lists RAR segments like "asdfghjklzxcvbnm yEnc 8" with other segments shown in the "Subject:" area having the exact same random letter/number/character name [everything from the yEnc backward] but with a different number after, usually in order.
However, when they are downloaded, every single one has a different random name, with NONE of them having a file type extension (i.e. none have either ".RAR" or ".PAR2" after them).
Newsbin has no clue what to do with them. And, until tonight, I had not figured out any way to actually access any part of them. [What I ultimately tried was to figure out, by size, which was likely to be a larger PAR2 file, add ".PAR2" to the end, and use QuickPar to figure out the parts and rename them. However, this didn't fully work... only some of the files were renamed, while others were ignored, despite being from the same NZB and being identical in size to the ones that were renamed.]
I can e-mail an NZB I tried this with, or link to an NZB that does this, if my description is unclear or if you want to test it yourself.