"What happens when an NFL ball goes into the stands? In this case, a lawsuit"
NFL game balls going into the crowd aren’t as common as in baseball, where home run balls and foul balls regularly make their way into the stands. But still, it happens, whether on an errant throw or in celebration by a player. And, per the NFL, no policy requires a game ball to be returned.
If the NFL team and/or stadium personnel wrongly told the fan that he couldn't keep the ball (and instead roughed him up and threatened him with arrest), then they deserve to lose a big lawsuit
