I recall in the Zimmerman case, originally the grand jury did not see evidence to indict. So they went to another grand jury until they got the desired answer.
The very same tactic was used by Travis County, TX DA Ronnie Earle against Republican House Majority Leader Tom Delay. It took Earle seven grand juries to secure an indictment against Delay. The seventh finally did Earle's bidding, and after a long court battle Delay was convicted...on charges that, after years of legal delays, were dismissed as a farce -- what Delay had done wasn't even illegal! -- but Earle had forced Delay out of politics, which was his true aim.