A large percentage of those who use a GriGri to belay a leader use the device improperly. Many climbers have the bad habit of holding down the locking mechanism while feeding out rope to a leader. If the leader falls while the mechanism is disengaged, it is possible that he could deck.
Petzl recently posted the following instructional video on their website. The video identifies two techniques that might be employed to safely and effectively belay a leader.