I noticed a problem while unraring a file I re-downloaded, by joining it again (from NLSS) into an existing collection.
(This, because I saw some missing blocks - maybe damaged NZB or something, I don't know.)
Anyway, the second download of the file, didn't overwrite the (damaged) existing file, but added "x" after the extension (example.rar -> example.rarx).
Par2-checking okay-ed the download after that (no repair needed), but unrar used the damaged file... resulting in a failed extraction.
I think the par2-checking forgot to rename the better file, or couldn't handle the fact that a file already existed with the same name (even though it was a useless, damaged file).