The late Brittanie Cecil (1988-2002) has the unfortunate distinction of being the first spectator to be killed by a hockey puck at an NHL game. On Saturday, March 16, 2002, she was hit in the temple by a puck at a Columbus Blue Jackets game against the Calgary Flames at Nationwide Arena in Columbus, Ohio, and died three days later, on Tuesday, March 19, 2002. She was 13 years old.
After the puck was shot by Blue Jackets forward Espen Knutsen, it deflected off an opposing player, and then bounced into the stands at about 80MPH, where it struck another spectator before nailing Brittanie's left temple. While she was able to walk out of the arena under her own power, she nevertheless died later of a severely bruised cranial artery.
The NHL itself and many arena executives have stepped up their safety awareness protocols, and are considering the installation of puck-catching nets in all arenas to protect spectators.
UPDATE! November 1, 2002: The start of the 2002-2003 NHL season brought the installation of black nylon netting in every NHL arena. It encircles the ice from approximately the right face-off circle, back behind the goal, and then ends at the left face-off circle, on both sides of the ice. If a puck touches the net, it's out of play, even if it bounces back onto the ice, and the player who put it in the netting is assessed a two-minute delay of game minor penalty.
It's barely visible in television broadcasts, and apparently hard to see from the seats in any of the arenas in which it has been installed, though I have not yet been able to verify this since I live in a city with no NHL team.
Anyway, it's saving lives. Pity the NHL suits didn't consider installing it until someone was killed.