In the real old days of hockey there was nothing above the boards to protect the fans from errant pucks and skates, or to protect players from fans. Eventually different types of mesh, usually chicken wire, was hung up, but was far from perfect.
Just after the Christmas break during the 1946-47 season, Toronto's Maple Leaf Gardens became the first team to install Plexiglas, also known back then as Herculite.
The "thermoplastic synthetic resin" was invented 17 years earlier by McGill University research student William Chalmers. Chalmers "unbreakable glass" of course is used in every hockey rink in the NHL and virtually all modern rinks in the world.
But as we see a few times in every NHL season, Plexiglas is not as unbreakable as Chalmers claimed. Which begs the question - when was the first time a NHL player broke the glass?
The answer appears to be in 1949, back at Maple Leaf Gardens. On December 14, 1949 Toronto defenseman Bill Juzda hammered Rocket Richard into the glass skates first. Ouch! You can bet that's what fans in the front row thought too. The glass exploded into a million pieces, cutting and bruising 5 fans, though none seriously. Richard was also fine.
YouTube doesn't have that one on film, but here's my favorite memory of broken hockey glass: