I've been talking to "Jimmy" at iTunes customer support for over a month now. My HD crashed and I lost everything. Jimmy was nice enough to allow me to download everything 1 more time. The reason we have been talking for a month is:
1. The first time, he stuck ALL 650 things in my pending downloads folder. iTunes crashed at least 50 times in a week and I barely downloaded a Gig in that time. I was also getting tons of different errors including this one.
2. So, he started putting in smaller batches and all was well - until I started getting this error message again yesterday.
3. Jimmy (who works for Apple iTunes) said that it was happening because of a bad file. So he cleared my queue again and put a smaller batch back in. Apparently, he is correct because the first batch downloaded fine. The new small batch he put in is AGAIN giving me the error message which means the bad file is 1 of the 39 in my queue.
Answer - contact customer support - they need to clear your pending downloads and put them back in which usually corrects the bad file transfer (happens usually with more than 2-3 purchases/downloads at a time).
Also, there are MANY other threads pertaining to this error. If you look for them on Google, the first one has an alternate solution about deleting your Downloads folder in iTunes. You can try it, but it did not work for me - also supports Jimmy's reason.
Hope this helps!
Contact iTunes here:
http://www.apple.com/support/itunes/store/download/