Twice that we know of, anyway. According to the Swamp (via Memeorandum):

Today, FOX News host Gregg Jarrett was talking about Republican Sarah Palin's book tour and the crowd she is drawing at the start of it -- no small turnout, with some 1,500 people lining up early this morning for a chance to get into this evening's premier book-signing for Going Rogue in Grand Rapids.

"Sarah Palin continuing to draw huge crowds while she's promoting her brand new book,'' FOX's Jarrett told his viewers. "Take a look at -- these are some of the pictures just coming into us... The lines earlier had formed this morning.''

But it turns out that Happening Now had pulled some video of something that happened last year: Displaying video today from Palin's campaign for the vice presidency, on the ticket with the GOP's Sen. John McCain -- which also drew considerable crowds, as shown today in video of a smiling Palin before an adoring campaign crowd.

Recall that last week, as Joel noted here, The Daily Show pointed out how Sean Hannity's program used old B-roll to apparently distort the crowd size of the Nov. 5 "Kill the Bill" tea party event on the Washington Mall. Hannity apologized (more or less). I didn't quite believe Fox's excuses, but I didn't quite see malice aforethought, either.

This, however, is embarrassing and beyond sloppy, even if sloppiness turns out to be the root cause. Shenanigans of this sort lend credence to Fox News' conspiracy-minded critics, of course. It also further validates the Bradbury Rule.

Update: Fox News explains the different footage was a control room gaffe. As much as it pains me to link to Think Progress, you can watch the video of the on-air apology there.