During Game 4, while many Bruins players and Panthers were in fights and scrums, Panthers forward and superstar Matthew Tkachuk decided to challenge Bruins goaltender Linus Ullmark to a fight in which Ullmark dropped his gloves and accepted the challenge.
It was quickly disbanded by the linesman and the referee as it's not too often you see a goaltender challenge a star forward. However, with the heated energy in the building and with so many fights and penalties being assessed it doesn't surprise me that this was going to happen.