Then no par2 files would be queued to download while it is verifying said file.
During that time all files in the nzb would appear to be paused or downloaded. If autocleanup runs at that time (automatically or manually) it will delete the nzb, despite the fact that the par2 data files are still needed to repair the files downloaded.
This happens if a large file is the last data file to be downloaded (say, a 700MB AVI for example). But might also occur in other circumstances.
Basically the cleanup routine should check to see that all the files verify correctly on par2 before cleaning them up.
