Buster Posey raced from second to third.
But the ball ricocheted right back to Rosario, who spun and nailed Posey by several feet.
"That was awesome," Chatwood said after the Rockies' 10-2 rout Saturday night. "That was completely my fault. We had just gone over the signs and I completely whiffed it. So, for that to work out like it did was pretty helpful.
"I crossed him up pretty good. But it worked out."
Just like the rest of the night did for the Rockies, who capitalized on three more errors by the mistake-prone Giants.
The wall was moved up six feet during the offseason when the Rockies added a walkway in front of the first row of box seats behind the plate, and that surely made a difference on the big play that helped thwart a big second inning by San Francisco.
"He crossed me up, but that's OK," Rosario said. "I got lucky the ball hit the wall and come back fast to me."
Chatwood (2-0) weaved his way in and out of several more sticky situations, allowing seven hits and four walks in 5 2/3 innings -- but he gave up just one run before hitting the showers after a 99-pitch performance.
Promoted from Triple-A Colorado Springs before the game for his second stint with the Rockies, Chatwood helped himself out with an RBI single off Tim Lincecum, too.
"I enjoy it, it's fun," Chatwood said of hitting.
Fielding has been a nightmare for the Giants on this trip.
"We just came off a really good homestand and everything's that hit is either not at us or we're having some difficulties handling tough balls. It's a funk," Giants right fielder Hunter Pence said.
In addition to his nifty defensive play to thwart a big second inning, Rosario drove in three runs for the Rockies, who beat the Giants for the second straight time after snapping a 10-game losing streak to them with a 10-9 win one night earlier.
Lincecum (3-3) committed a costly throwing error that led to a three-run third inning and also had a balk when he slipped off the mound that led to another run in the fourth before Rosario took him deep for his ninth homer in the fifth.
The Giants' defensive slump continued despite a team meeting before the game in which manager Bruce Bochy addressed San Francisco's poor play in the field.
Then, he watched the Giants commit three more errors, giving them a dozen in the first five games of their six-game trip that wraps up Sunday at Coors Field.
Posey airmailed a throw into center field on Carlos Gonzalez's stolen base attempt in the first inning. Gonzalez trotted to third but he was stranded there when Lincecum got Troy Tulowitzki to ground out to short to end the inning.
In the top of the second inning, Posey tried to advance to third on a pitch that got past Rosario only to be out by several feet. Brandon Belt followed with a two-out double to the right-field corner.
Gregor Blanco drove in Belt with a bloop single to center, and Brandon Crawford drew an intentional walk before Lincecum was caught looking at the third strike, thwarting what could have been a bigger inning.
The Giants' second error proved more costly as Lincecum's bad throw on a comebacker led to a big third inning for Colorado.
"It was just frustrating," Lincecum said. "Just not consistently hitting spots, leaving balls up. Kicking the ball around. That was probably the biggest frustrating thing for myself when I knew I could have made that play. After that I didn't collect myself and concentrate. Kind of unraveled."
Dexter Fowler doubled down the left-field line, scoring Charlie Blackmon from second to tie it at 1. Fowler scored when Lincecum fielded Josh Rutledge's chopper and threw the ball into right field. Rutledge ended up at third base and trotted home on Gonzalez's groundout.
"Trying to make something there out of what's not possible," Bochy said. "That's what we've got to stop doing, compounding the problem. We've done it a few too many times. It might have caused him to lose a little focus."
Lincecum said the Giants are pressing on defense.
"People try to do too much in tough situations because we want to make that big play. That's who we are," Lincecum said. "When it doesn't get made, it just adds to that frustration."
The last time the Giants committed 12 errors in a five-game span was April 18-22, 1996, according to the team.
"Errors aren't always mental," Lincecum said. "You try to find the silver lining in the effort. It's not for lack of effort. Everyone's hustling, everyone's trying to make that play, that's why it's tough."
Lincecum slipped off the rubber while pitching to Blackmon in the fourth, resulting in a balk that sent Nolan Arenado to second base. He took third when Blackmon flied out to center and scored on Chatwood's single back up the middle, making it 4-1.
"My spike slipped on the mound," Lincecum said. "It's happened twice here already."
The Giants put runners at the corners in the fifth but Posey grounded out to the mound to end the threat, and Rosario's two-run homer, his ninth, made it 6-1 in the fifth.
All of the six runs Lincecum allowed in five innings were earned. He gave up seven hits and walked a pair.
Rosario's 60-foot dribbler down the third-base line off Jean Machi in the seventh drove in Fowler from third to make it 7-1.
Crawford's RBI single made it 7-2 in the eighth, but he committed San Francisco's third error in the bottom half of the inning when he took his eye off a throw back into the infield, allowing Fowler to advance to second following an RBI single.
Tulowitzki added a two-run single off George Kontos to make it 10-2 ... Josh Outman threw two effective innings of relief and Wilton Lopez got the final four outs. ... Rockies OF Michael Cuddyer says he doubts his neck will bother him again this season after getting an epidural that relieved pain and stiffness. He returned to throwing and taking batting practice Saturday. ... The Giants and Rockies wrap up their four-game series Sunday with LHP Barry Zito (3-2) and RHP Juan Nicasio (3-1) squaring off.