Two troubling trends surfaced: For the second time within a week, the Giants followed an emotionally charged win with a flat defeat. For the umpteenth time this season, they were destroyed by one or two mistakes.

Both are tied to an offense too easily squelched by middling starters such as Mike Pelfrey, who held the Giants to two runs in 71/3 innings Sunday.

This game was settled on the final pitch as Daniel Murphy singled to right on a hanging 2-2 curveball from Jeremy Affeldt, with one out in the ninth, to score Jeff Francoeur and send 38,793 fans home. However, as often happens, the Giants left themselves open to a late loss by failing to capitalize earlier.

"We were missing a hit," manager Bruce Bochy said. "They got it in the end."

The Giants made a nice run in the eighth inning. They tied the game on back-to-back doubles by Eugenio Velez and Randy Winn, a good sign for both. Velez was 0-for-11 and 2-for-23 before a fifth-inning RBI single. Winn, who has been relegated to the bench, took Pelfrey to 3-2 before smacking the ball to right-center.

Pinch-hitter Ryan Garko had a shot to put the Giants ahead with two outs against Pedro Feliciano. This was precisely the situation the Giants had in mind when they acquired Garko, who hits lefties well. He had a good at-bat, too, and smoked a 3-2 pitch to the right side, but right to second baseman Luis Castillo.

Winn's hit did spare Jonathan Sanchez a defeat. Both runs against him in seven innings scored on a rare homer by Castillo, who had not hit one since May 30, 2008, a span of 487 at-bats. But Castillo should not have been up in the fifth inning.

Fred Lewis, whose defense has been a problem all season, could not hang onto an Angel Pagan drive that he reached after a lengthy run. Pagan was credited with a double.

"We're not going to be pointing fingers," Bochy said in Lewis' defense. "He ran a long way. He didn't cost us a run by dropping it. Sanchez just made a mistake to the next hitter."

Again, these are mistakes the Giants cannot afford because they can't hit their way out of them.

"I think that's just kind of the way our season has been going," Winn said. "We realize we need to play good baseball. We can't afford to make a whole lot of mistakes defensively, offensively or on the bases."

The second-guessers will argue that Bochy made a mistake by summoning Affeldt instead of walking Murphy intentionally and allowing Sergio Romo to continue his inning against right-handed hitters Omir Santos and Anderson Hernandez.

Bochy noted that Murphy and Santos were hitting .222 and .226 against left-handers.

"You look at the numbers and what they're hitting against lefties, and I've got one of the best lefties in the game in the 'pen, a power arm," Bochy said. "I went with matchups."

Affeldt is going through his first rough patch with the Giants, since the end of his 28-inning scoreless streak. Twice recently he was pulled after walking his only hitter. He has frozen many hitters with the curveball he tried to throw Murphy.

"You can't have scoreless innings your whole year," Affeldt said. "Yeah, I've gotten behind some guys earlier and I'm getting into hitters' counts, which I did a pretty good job not doing. Today, I hung a curveball and he hit it. I need to execute a little better in that situation."

E-mail Henry Schulman at hschulman@sfchronicle.com.

This article appeared on page B - 1 of the San Francisco Chronicle