Pacioretty put a wrist shot past Tim Thomas into the top left corner and then pushed Bruins captain Zdeno Chara as he skated behind him, touching off a wild scene in the right corner.
Montreal, which played its 3,000th NHL home game, tied it with two goals late in the third. Scott Gomez scored a power-play goal at 17:38 to ruin Thomas's shutout bid and draw the Canadiens to 2-1.
Brian Gionta scored the tying goal with 47.7 seconds remaining in regulation, which brought on a huge ovation from the sold-out Bell Centre crowd.
Patrice Bergeron scored a pair of second-period goals for Boston, which killed a penalty in overtime after Blake Wheeler was called for hooking 10 seconds into the extra period.
Carey Price stopped 27 shots for Montreal.
Thomas made 39 saves for the Bruins, who are tied with Montreal for the Northeast Division lead with 49 points.
Bergeron scored his ninth goal 4:18 into the second. He added his second of the game — his 10th — at 12:27.
Montreal had an apparent goal by Mathieu Darche disallowed 7:57 into the third. Referee Kevin Pollock initially pointed to the net after Darche batted a rebound of James Wisniewski's point shot out of the air and into the net off Thomas' left glove.
Linesman Pierre Champoux immediately skated over to consult with Pollock, who then waved off the goal as he skated toward the official scorer with partner Stephane Auger. A video review upheld the referee's revised call.
Canadiens left wing Michael Cammalleri left the game in the second period with an undisclosed injury. He did not return.
Notes
Bruins right wing Nathan Horton missed his first game of the season because of an undisclosed injury. … The Canadiens have 1,833 wins, 748 losses, 382 ties and 37 non-regulation losses at home since joining the NHL as a charter franchise in 1917-18. … Canadiens defenseman Roman Hamrlik returned after missing Thursday's shootout win over Pittsburgh with an undisclosed injury.