One thing that a lot of people noticed was that Intrade's odds often seemed strangely tilted towards the GOP. Compared to better predictors like the polls and various polling aggregators, Intrade usually gave Obama significantly worse chances for re-election.
Well, it turns out that some anonymous trader was systematically spending millions to boost Romney's odds on Intrade:
Two economists who studied the data offer various rationales for the trader’s aggressive wagering on Mr. Romney in the final two weeks of the campaign. The anonymous trader placed 1.2 million pro-Romney contracts, some of which were actually in the form of bets against a Barack Obama victory.
The most plausible reason for the betting, the authors conclude, is that “this trader could have been attempting to manipulate beliefs about the odds of victory in an attempt to boost fundraising, campaign morale, and turnout.” [...]
“It is worth knowing that a highly visible market that drove many a media narrative could be manipulated at a cost less than that of a primetime television commercial,” the authors write.
This lone trader was responsible for one-third of all the pro-Romney bets on Intrade during the last two weeks of the campaign.