George Zimmerman's defense attorney Mark O'Mara walks a lap top up to a state witness and has her read a post from her Facebook Wall that read in part, "prosecute the killer of our son 17-year-old Trayvon Martin" in reference to a Change.org petition. 

I asked former felony prosecutor David Weinstein about that moment. He told me over the phone that you can expect the defense to remind jurors that she is not an impartial witness during closing arguments. Also: "If the defense can prove that the prosecution knew about the Facebook post and/or helped the witness to conceal her bias, then they have grounds to move for a mistrial. However, not all motions for mistrial are granted or necessarily result in a “do-over” or are a complete bar to a mistrial."

This is the second time Facebook has surfaced during the trial. A "pro-Trayvon" post by one potential juror had him dismissed early on in the process. In part because he had claimed not have posted anything about the case. Here's a look back at that story: 

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