The New York Rangers had just tied Game 2 at 1-1 in the second period when Rangers winger Derick Brassard broke in for a go-ahead goal.
But Penguins coach Mike Sullivan challenged the goal on the basis that Brassard was offsides as he entered the offensive zone. The league had a couple of angles to work with.
Here’s the NHL’s new blue line camera at work pic.twitter.com/mOh1YkySug— Pete Blackburn (@PeteBlackburn) April 16, 2016
Onside; Brassard's goal will stand. 2-1 Rangers pic.twitter.com/JmUbJrQRUy— Ryan Ohanesian (@ryanohan) April 16, 2016
Remember, the offsides rule is clear: a player is offsides when "both skates are completely over the leading edge of the blue line involved in the play," and a player is on sides "when either of his skates are in contact with, or on his own side of the line, at the instant the puck completely crosses the leading edge of the blue line."
Based on the angles, it was an extremely close call. The officials upheld the goal call, and the Rangers went on to take a 3-1 lead by the end of the period.
The NHL allowed coach's challenges this season, including those for offsides goals. They even installed special blue line cameras to aid in this, and it's already become a point of controversy just days into the playoffs.