Cormier's errant throw to third in the ninth inning allowed Garrett Atkins to score the go-ahead run, and the Rockies beat the Diamondbacks 6-4 Friday night for their fourth win in five games.
"Take a nap, make a mistake and we have a chance to take advantage of you," Colorado manager Clint Hurdle said. "We can come back and beat a ballclub. The road's been the challenge."
The Rockies have won three of their last four on the road but still are 13-41 away from Coors Field.
Atkins led off the ninth with a double to center and pinch-hitter Eddy Garabito laid down a sacrifice bunt between home plate and the mound. Cormier (7-2), the fourth Arizona pitcher, quickly pounced on the ball but threw wide to Alex Cintron at third, allowing Atkins to score.
"They played right in our hands, got (Garabito) to a good count," Cormier said. "With two strikes, most likely he's going to bunt back to you and usually I can control it and throw him out. That's my strength, but I just threw it wide."
Garabito eventually scored on a sacrifice fly by Cory Sullivan.
Mike DeJean (5-2) pitched a perfect eighth for the win. Brian Fuentes finished for his 19th save in 22 chances.
The Diamondbacks were four outs from remaining a game behind the San Diego Padres in the NL West, but Matt Holliday tied the game at 4-all in the eighth with a two-out, broken-bat homer to left-center field off Greg Aquino.
"I thought I was going to drive the ball but then I felt the bat give way," said Holliday, who was left holding a six-inch stump in his hand. "I was surprised. I wasn't quite sure what to do with (the handle.)"
Omar Quintanilla started Colorado's eighth-inning rally with a one-out single off Buddy Groom. Aquino came in and struck out pinch-hitter Luis A. Gonzalez, but Holliday hit a 426-foot shot to left-center, his 10th, to tie the game as the barrel of his splintered bat skidded into the fence in front of the Diamondbacks' dugout.
"We're one pitch away and Holliday gets a breaking ball in the middle of the plate and hit it pretty good," Arizona manager Bob Melvin said. "I guess his bat went all over the place but he still hit it pretty good."
The Diamondbacks went ahead with three runs in the third inning.
Chad Tracy hit a leadoff homer, his 15th, to tie it at 2-2, and Koyie Hill doubled one out later. David Cortes retired pinch-hitter Conor Jackson and went to 2-0 on Craig Counsell before intentionally walking him.
Royce Clayton followed with a triple into the left-field corner, scoring Hill and Jackson to make it 4-2.
Colorado took a 1-0 lead in the fourth against Brandon Webb when Quintanilla scored on a groundout by Holliday. The Diamondbacks tied it in the sixth on Tony Clark's sacrifice fly.
Larry Bigbie scored on Webb's second wild pitch of the game in the top of the seventh for a 2-1 Rockies lead.
Webb allowed just three hits, struck out a season-high 10 and walked one.
"That was the best I felt in I don't even know how long," Webb said. "My arm felt really alive and the ball felt great coming out. I was able to locate my pitches whenever I wanted."
Colorado starter Aaron Cook, making his second start since coming off the 60-day disabled list July 30, scattered four hits with two walks and three strikeouts in six innings.
"I felt like I needed to go out there and have a good start after the last one," said Cook, who allowed seven runs on 11 hits in 4 1/3 innings in his first outing back. "All the bullpen work and the work on my mechanics are paying off."
Before the game, the Diamondbacks announced they had signed Clark to a two-year extension. Terms of the deal were not disclosed. Clark is hitting .329 and is second on the team with 18 home runs. His 15 pinch-hit RBI are one short of the club's single-season record. ... Colorado claimed RHP Sun-Woo Kim off waivers from the Washington Nationals on Friday. Kim was 1-2 with a 6.14 ERA in 12 games for the Nationals, including two starts.