I recently switched form SickBeard to Sonarr and had the category in sonarr set incorrectly to 'tv'. I had to setup sonarr to use a new category 'dronetv' due to another bug where it seems no matter what config changes to make it seems to complain that SickBeard and Sonarr are using a duplicate category of 'tv' if you try to get sonarr using that....
But I digress.. I had sonarr add a bunch of downloads, but I had forgotten to set the category in sonarr that it uses when sending the nzb to nzbget. As a result post processing failed. Editing the Post Processing scripts to add in nzbToNzbDrone and telling the hostory record to post process again, fails since the category is wrong and I don't have sufficient time to edit it before the processing starts.
The only way I have been able to resolve is by telling it to download again, and then changing the category after it is in the download queue.
Allowing me to change the category before I tell it to Post Process again would result in less time, bandwidth savings and successful post processing.
I suspect this might also occur on occasion when manually adding in nzbs and not noticing that you left the category blank or wrong.