The controversial part of the IPCC report was authored by a relative handful of scientists, who were referencing their own work and as lead authors were given the power to reject criticism of their own work!
Once it published it's part of the literature, and it's the "authoritative report"
You think every scientist referencing the IPCC report is checking it first?
Very recently, a paper was published in J. Climate that documented a flaw in the ERA-40 analysis which "leads to significantly exaggerated warming in the Arctic mid- to lower troposphere" This ERA-40 analysis has been referenced over 2000 times in literature, yet nobody found this problem until now. It didn't take a large coordinated conspiracy to get it wrong. It just took a flawed analysis by a few people that was accepted and not checked to invalidate 2000+ papers.
All you need is a few people to get it into the literature, which is much easier when the same few are "peer reviewing" their own work, and it spreads.
Eventually, actual science catches up and problems are revealed. But it's a slow process and even slower when it's as much a divisive political issue as a scientific issue, as AGW certainly is.
From Phil Jones To: Michael Mann (Pennsylvania State University). July 8, 2004
"I can't see either of these papers being in the next IPCC report. Kevin and I will keep them out somehow — even if we have to redefine what the peer-review literature is!"
(Phil Jones and Kevin Trenberth - Both IPCC Lead Authors in the above mentioned IPCC report. You don't need thousands of willing conspirators when the power to shape the report their way is given to only a few)