I have just noticed that if the program attempts to unrar to a drive that is running out of space, the program will unrar as much as it can in the free space but once the limit is reached, it appears to treat the unrar as being successful and then deletes the parts. No warning is given so the only way to 'fix' it is to clear enough space on the drive and do the download again.
I'm not sure if this has already been reported but hope this post will be useful.