It seemed especially true when a pair of second-period power-play goals gave Boston a 2-0 lead against a New York squad that had scored five goals combined in the series.

But all of a sudden, the Bruins started making unusual mistakes that cost them Game 4 at Madison Square Garden. The Rangers kept their season alive with a 4-3 win earned on Chris Kreider's goal 7:03 into overtime.

The Bruins lead the best-of-7 series 3-1 with Game 5 on Saturday at TD Garden (NBCSN, RDS, TSN).

Boston's mistakes Thursday were made by it best players.

It started with goaltender Tuukka Rask, who had anchored a Boston defense that was airtight through three games. With Boston up two and the hometown energy completely sapped from MSG, Rask appeared to trip over his own feet as Rangers forward Carl Hagelin skated toward him. The stumble took Rask out of the play, but he tried in vain to swat at Hagelin's weak shot with his stick, missing as it headed into the net.

With one bizarre play, the Rangers had new life.

"It happens to me twice a year in practice, maybe. I've got to be more focused, I think. Just a tough mistake. It looks pretty bad on TV, I'll bet," Rask said. "It was just sloppy. Then you still have a second to decide if you're going to scramble or get the paddle down or just try to whack it away. I tried to whack it away."

After Boston took a 2-1 lead into the third period, it was another costly error that allowed New York to tie the game. This one came from captain Zdeno Chara, arguably the sturdiest and most consistent defenseman in the world.

With Chara handling the puck behind his own net, Rangers forward Derek Stepan swooped behind him and stole possession. Before the Bruins captain could realize what happened, Stepan successfully tucked a wraparound past Rask and tied the game 1:15 into the third.

"I wasn't aware. I've got to make sure I take a look," Chara said. "We made some mistakes and they capitalized on those. We need to make sure we clear that part of the game and get ready for the next game."

To its credit, Boston was able to retake the lead when Tyler Seguin tucked home his rebound past Rangers goaltender Henrik Lundqvist just after Ryan McDonagh's interference penalty expired.

But it wouldn't be long before another costly mistake sapped Boston of the momentum.

With 11:05 remaining in the third, as Shawn Thornton approached the bench to complete his shift, Seguin jumped on the ice prematurely. Thornton was still in the play when teammate Chris Kelly grabbed the puck and Boston was whistled for too many men on the ice.

"I think it was my fault. I saw [Thornton] coming and I hopped over. He stayed on for an extra second," Seguin said. "It was a close call and they made us pay for it."

Sixty-five seconds after Seguin began serving the two-minute minor, the Bruins' penalty kill allowed its first goal of the series, giving life to a Rangers unit that had not scored a power-play goal in 23 straight attempts.

The mistakes didn't go unnoticed by the Bruins, who will look to minimize such costly plays when they return to Boston for Game 5.

"It's a game of mistakes. Every team makes mistakes, every player makes mistakes. You just need to learn from them and move forward," Rask said. "I don't think a couple of mistakes are going to make us a bad hockey team. It's just something that happens sometimes and you've got to shake it off and move on."

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