The first 10 minutes of Season Four.
From Broadcasting & Cable: "Exclusive clips also create a thrill, so much so that Sera Gamble, executive story editor for The CW's Supernatural, recently overheard executive producers Eric Kripke and Robert Singer debate what to show at their panel. Kripke argued hard for the first 10 minutes of this fall's season four to please the fans. “They deserve it,” he said. But Singer nixed the idea partly because “10 minutes later, everyone on planet Earth will know about it” via the blogosphere."