Tony Cascio scored on a counter-attack in added time as the Rapids pulled off a comeback 4-3 victory over the Impact on Saturday night.
It was the first home defeat of the season for Montreal, which wasted leads of 2-1 and 3-2 in the second half. It was also their first loss to a Western Conference opponent.
"It wasn't our greatest game," said Impact midfielder Patrice Bernier.
Montreal had everyone up field for a corner kick when midfielder Davy Arnaud was stripped of the ball and the Rapids counter attacked. Chris Klute's attempt was blocked, but the ball bounced to Cascio, whose shot went off Bernier and past goalie Troy Perkins for the game-winner.
The sudden reversal stunned the Impact and the 20,383 fans in the seats.
"I just went in to help (Klute)," said Cascio, who had entered the game as a substitute in the final minute of regular time and ended up with his first goal of the season. "No one expects it to deflect back to you, but I was in the right place at the right time and I just shot it."
Another late substitute, Deshorn Brown, had tied the game in the 77th minute.
Atiba Harris and Dillon Powers also scored for the Rapids (6-7-5), who ended a three-game losing run with their highest offensive output of the season.
Daniele Paponi, with his first two MLS goals, and Hassoun Camara scored for Eastern Conference-leading Montreal (9-4-2), who are now 6-1-1 at home.
Arnaud was in his first MLS game in nearly two months after missing nine games with a concussion.
The veteran midfielder's hustle helped set up Paponi's go-ahead goal in the 73rd minute, but it was his turnover that led to Colorado's game-winning goal.
"I was trying to come inside to make a play and I got caught," he said. "It cost us the last goal.
"I take full responsibility for that."
The Impact may have been wiser to play cautiously in added time and settle for the draw, but they felt they could go for the win and it backfired.
"We feel like we can win every game at home," added Arnaud. "That last corner, we had numbers forward and maybe we weren't organized like we should have been, but we want to win the game."
It was a game of defensive breakdowns at both ends for two of the better defensive clubs in MLS.
"We made a lot of mistakes that gave them opportunities," said Bernier. "I won't take credit away from them.
"They played their game. But since we were doing so well offensively we thought we could go and score four or five. Sometimes we have to manage the game better so the defence can have a breather. But it happens. We weren't going to play 34 great games (this season)."
It was the second time in as many meetings that Colorado has pulled off a late win over Montreal. In Denver last season, the Rapids were down one man but still managed a 3-2 victory.
The Rapids played defensively to start, and drew some boos by dallying and slowing play whenever possible, but they got the opening goal off a free kick in the 24th minute.
Lanky former Vancouver Whitecap Harris stayed at the edge of the crowd in front of Perkins and headed Powers' kick inside the far post for his team-leading fifth of the season.
Camara got the equalizer in the 38th minute while on his back, as Andrea Pisanu's corner kick ping-ponged inside the Colorado box and the Montreal fullback got a foot on it with a semi-bicycle kick and it went in.
Edson Buddle had just tipped a Harris pass off the crossbar when Marco Di Vaio run up the middle on a counter-attack and fed Paponi at the edge of the penalty area. The Italian cut inside and bent a shot inside the post in the 42nd for his first MLS goal.
Di Vaio, who leads the team with 10 goals, got his first assist of the season.
The Impact defence was caught standing around as Powers took a pass from Nathan Sturgis and slid it in along the ground in the 59th.
Arnaud kept a Felipe Martins' pass from crossing the end line and chipped it in front for Paponi to push past Clint Irwin for a 3-2 lead in the 73rd.
But Brown, only three minutes after replacing Buddle, ran straight up the middle to fire a Sturgis pass behind Perkins in the 77th.
"It's what happens at the end that's important and it was a bad end for us," said Impact coach Marco Schallibaum. "That's football.
"Now we have Toronto and that's another game."
The Impact have their second MLS meeting of the season with TFC Wednesday night in Toronto.
For Rapids coach Oscar Pareja, the game was "a hard test against a good rival. And to be honest, I thought our team deserved the victory."
Notes: Arnaud started on the bench but went in for Pisanu in the 58th. . . Sanna Nyassi (hamstring) sat out. . . Colorado's Shane O'Neill and Dillon Serna are away with the U.S. under-20 squad, while midfielder Martin Rivero, defender Diego Calderon and goalie Matt Pickens sat out with injuries.