Petry won it for the Canadiens when he batted the rebound of Max Domi's shot out of the air past Rask. It was Petry's 10th goal of the season.
"Well clearly right now it [stinks], especially when it happens that quick," Bruins coach Bruce Cassidy said. "It's a pretty straight-forward rush. We folded instead of challenging the puck carrier, so that started a domino effect and we lost coverage in front."
Marchand's 17th goal gave the Bruins a 1-0 lead at 14:09 of the first period during a 4-on-4 situation. After taking Patrice Bergeron's pass in the neutral zone, Marchand gained the Canadiens end with speed and beat Price with a wrist shot from the left dot inside the far post.
Gallagher tied the score 1-1 with a tip-in at 18:27. It was his 18th of the season and came after Phillip Danault beat Marchand in a face-off back to Petry, whose wrist shot from the left point went off Gallagher's stick.
Byron gave the Canadiens a 2-1 lead when he scored shorthanded at 17:09 of the second period. Byron poked the puck away from Bergeron at Montreal's blue line and took off before making a move to the middle of the Boston zone and scoring on a backhand high to Rask's glove side.
Krejci's 10th of the season tied it with a 6-on-4 power-play goal after Rask was pulled for an extra attacker. Krejci's wrist shot from the top of the right circle went off Jordie Benn in traffic in front of the net.
"Just try to shoot, you know, you always practice that in practices," Krejci said. "But it doesn't happen if [Jake DeBrusk] doesn't do a good job screening the goalie. So obviously a big goal for us, and I think we had the momentum on our side. We just couldn't get it done."