Thats where par2 come in, to repair any damaged files
(due to missing parts or botched posting, or whatever, this is usenet after all.)
http://www.slyck.com/Newsgroups_Guide_PAR_PAR2_Files <--- for some theory
in a few words, either have altbinz autorepair
-or-
google quickpar,
grab it ,
install it,
....before you remove the paused entry from altbinz (the paused entry has the .par2 recovery volumes that weren't downloaded)
check everything you download by opening the small .par2 file (or just any .par2 file that you downloaded).
if it sais it's complete, you're done.
if it sais it needs x volumes to repair then:
unpause x or more volumes from the queue,
wait for them to d/l,
run the .par2 again and it'll repair it.
(on an extra note, be careful not to have two quickpar instances running on the same parset)