Elias Pettersson was battling for the puck, when Walker Duerh tried to lift his stick which led to Pettersson's stick hitting Duerh's helmet. None of the two referees caught the play, it was ultimately the linesman that winded up making the call. Per the rule book, the linesman can only blow the play for a penalty worth at least a double minor, which was not the case.
The rest is history, Flames got on the power play, tied the game up, and took the two points home when Tyler Toffoli tallied the OT winner. Looking at it this way, it's hard to believe that the NHL isn't rigged when a game-changing call like this one occurs. Especially when the Flames are battling for a playoff spot:
Not only did the Officials fail to make the right call, but they weren't even able to give the proper explanation for it. Officiating has been horrendous this year and every time a coach took his anger to the media they ended up getting fined.