Recently there was a big mess because some folks signed an exclusive with a Taxi client but had outstanding non-exclusives on the same track(s).
Another thing that some folks may or may not know. If you sign a song to an exclusive deal, you cannot sign the no-vocal backing track to the song to a non-exclusive deal... at least not without permission. That first exclusive covers it all-- libraries need the backing track for mixes by end users and sometimes even to place as tracks to themselves.
