With runners on first and third and two outs, Anthony Rizzo rifled a ball under the legs and to the side of the glove of the sure-handed Belt, allowing Julio Borbon and Junior Lake to score and silence a shocked Giants crowd.
"It was just a routine ground ball," said Belt, who added that he didn't get his body down quickly enough to stop the ball from going between his legs and into the outfield. "I did what I've always done in that situation."
Sergio Romo's blown save was his first since June 15 and just his fourth of the season. The Cubs also scored two in the ninth off Romo at Wrigley Field on April 12 to win. Belt said he apologized to Romo on the mound after the misplay.
"[Romo] did everything he could to get out of it and it was very unfortunate I didn't come up with it," Belt said.
For a Giants club that has now fallen 10 games under .500 for the second time this season -- and is now eight games behind the first-place Dodgers -- it's another loss in a recent string of them, but this one might be the most disheartening, Bochy said.
"This was a tough one," he said. "It's always hard at home when you lose a game like this."
Following a four-game series in which the Reds shredded San Francisco pitching, Cain turned in his second consecutive effective outing to begin the second half. San Francisco's offense provided late timely hitting -- a rarity recently -- to take a temporary lead.
Trailing, 1-0, in the seventh, Jeff Francoeur's opposite-field single scored Pablo Sandoval and Hunter Pence to lift the Giants ahead, 2-1. The hit came on Edwin Jackson's 94th pitch of the evening and chased the Cubs starter from the game.
But the go-ahead play did not come without a price. When right fielder Nate Schierholtz's throw sailed left of catcher Welington Castillo, Belt charged from third to try to score the third run on the play. Jackson, who didn't immediately back up the play, caught the ball and tagged out Belt for the second out.
"When the ball passed, I didn't even see him," Belt said. "Fortunately for him, the ball bounced right to him and it was costly for us."
Jackson said he was initially out of position because he didn't want to run into Sandoval.
"By the time I could turn around, the ball was coming at me and right in front of me," Jackson said. "Sometimes the ball bounces in your favor, and sometimes it doesn't."
Bochy said the mental error is more difficult to accept than the fielding one in the ninth.
"We made a baserunning mistake that killed us," he said. "Bad judgment there and they got a big out.
"You're on third base with less than two outs and that's what you want. Just bad judgment."
Belt nearly redeemed himself in the bottom of the ninth when he lifted a first-pitch fastball to deep left-center field with a runner on, but the ball died just short of the warning track.
Cain tossed a season-high 115 pitches in seven innings in a stifling performance. He struck out seven and walked just one while scattering six hits. With the exception of the sixth, he overpowered the Cubs whenever they reached base, stranding runners in six of seven innings.
"I felt good," Cain said. "I just had a good feeling out of the [pregame tosses in the bullpen] early."
Before the rally, the Giants mustered just two hits off Jackson in the first six innings. His efficiency allowed him to throw just 75 pitches in that span before faltering the third time through the order.
Schierholtz handed the Cubs the lead when he plated the game's first run in the top of the sixth after doubling and scoring on a Starlin Castro single. Schierholtz drew a seven-pitch walk in the eighth, but Sandoval made a diving stop and threw out Darwin Barney with two outs and runners on second and third to preserve the lead.
With the loss, the Giants fell to 7-17 against the National League Central in 2013. San Francisco has fared miserably against opponents outside of the division, as the defeat dropped the club to 17-35 record against non-NL West teams.