Instead, Cain hit a game-tying double, the second of three consecutive doubles that helped the Royals rally for a 3-2 victory on Wednesday night -- their second straight in the series at Kauffman Stadium.
With the A's leading, 2-1, Eric Hosmer doubled with two outs in the seventh, and Cain followed with a ball to deep left field. Yoenis Cespedes made a leaping effort to catch it, but the ball hit off the wall and went for an RBI double. That tied things at 2 and chased McCarthy from the game.
Sean Doolittle relieved McCarthy and gave up another RBI double, this one on the 11th pitch of an at-bat to Chris Getz. Just like that, McCarthy's bid for his first win since June 19 was snuffed out, and sent him to his first loss since April 21.
"It's a bad loss for us across the board," McCarthy said. "These are the games that I think we've expected to win all year and been winning, and to give that up late like that is -- we need to do better than this. I'm not happy with myself, and there's a lot of things we could've done today to be better."
McCarthy was making his second start since returning from the DL after allowing three runs on three solo home runs on Friday. After allowing an unearned run in the first inning on a fielder's choice to Billy Butler, McCarthy cruised through the middle innings before running into trouble in the seventh. In all, McCarthy allowed three runs (two earned) on seven hits.
"I thought he pitched great," third baseman Josh Donaldson said. "He was really one pitch away from sealing the deal for us. He looked really good. They weren't taking very good swings on him all night. Sometimes you have to tip your hat to the other team."
McCarthy, understandably, was not as pleased.
"I don't like giving up runs," McCarthy said. "I don't like giving them up late in the game. I don't like doing it with two outs. I don't like doing it when I don't execute a pitch. There's just nothing about that I like."
McCarthy said afterward that the pitch to Cain stemmed from a communication mix-up between he and catcher Derek Norris. He said there was a misunderstanding on the location of the pitch.
Despite all of that, it looked, for a moment, as if Cespedes might have been able to save the day for McCarthy and the A's on the Cain line drive. Even the batter wasn't so sure he delivered the game-tying blow.
"I thought he was going to catch it," Cain said. "I was looking at him the entire time and it looked like he had a good bead on it, but I guess I got it up high enough to hit it off the top of the fence. And I'll take it."
The A's lost another tough battle in Doolittle vs. Getz. The Royals second baseman fell behind 0-2 in the count and then proceeded to see nine more pitches: five fouls, three balls and a double down the left-field line.
"Ideally you don't want to start 0-2," Getz said. "He kept on throwing his heater and showing his offspeed stuff, and he just kept on battling and battling and battling. It was one of those at-bats where you kind of get more comfortable as time goes on, once you've seen everything, and eventually you're fortunate enough to sneak one in there."
For much of the game, it was the A's on the positive end of things. Trailing 1-0 after one inning, Brandon Moss' RBI single tied the game in the top of the second, and Donaldson smashed a solo home run down the left-field line off Royals starter Will Smith in the top of the fourth to give the A's a one-run lead.
"[Smith] threw me a curveball down in the zone, and I was able to get under it a little bit," Donaldson said. "I didn't know if it was going to stay fair or not because it was kind of hooking. Fortunately for me it did."
"I was praying it would go foul but it didn't," Smith said. "That's my fault. He did his part -- he hit a good pitch. I thought I made a good pitch, it was down and he golfed it out so he did his part. He's a good hitter."
Still, A's manager Bob Melvin thought the team had other opportunities to add to the advantage. The A's had runners at the corners with one out three times, and they cashed in just once, on Moss' single.
"I think at times you press a little bit when you get the opportunities and you're not getting them in," Melvin said. "But it's not like we left 15 guys out there, one of those types of games. But we had some opportunities early on to pad our lead, and when you don't, it gets down to the end of the game and one hit can beat you or one hit can tie it. And that's what happened."
The loss clinched a second consecutive series loss for the A's, who hadn't dropped back-to-back series since the last time they came to Kansas City: the first three days of June.
"We just have to go out there and try to win a game [Thursday]. Can't get caught up in that," Melvin said. "We've won a lot of series here recently, and we lost a series in Chicago and now it's our job to make sure we get one out of this one here before we go home."