Joe Smith's wild pitch with two outs in the eighth inning allowed Coco Crisp to score the go-ahead run, and the Angels lost 2-1 to the Athletics to fall into a first-place tie with Oakland atop the division.
"Obviously these are two really good teams," pitcher C.J. Wilson said. "A lot of these games are one- two-run games. You lose a game like that you go back and think about a pitch you might have been able to throw better, or catch a break on offense."
Both clubs know this is far from over, with five meetings remaining before August ends and another three in late September.
"We can't get caught up in the standings or the playoff picture," A's closer Sean Doolittle said.
Luke Gregerson (3-2) pitched a perfect eighth, and Doolittle finished for his 20th save. Doolittle felt something in his right side nearer to his back, but was able to go through some twisting exercises and other postgame tests with no problem. He doesn't consider it serious.
The division co-leaders (76-52) have the most wins in baseball. The Angels lost their fifth straight game this season at the Oakland Coliseum.
On Monday in Boston, the Angels moved into sole possession of the division's top spot for the first time since May 15, 2011. Losing the first two games of this series has things all tied up again.
Jon Lester and Wilson had a nice pitchers' duel but left with no-decisions in a tie game.
Lester struck out seven in seven innings, including Mike Trout three times as the slugger's five-game home run streak at the Coliseum was snapped.
Lester bounced back from his first defeat in nearly 2 1/2 months and first since joining the A's from Boston at the trade deadline. The lefty threw a combined 18 pitches in the fifth and sixth innings, 14 for strikes. He then allowed three straight hits in the seventh, including a double by Howie Kendrick and Erick Aybar's RBI single that made it 1-1.
The A's have struck out Trout three or more times in one game six times for his career, most by any team.
"Maybe we were able to get him off his game a little bit, get him frustrated and to swing at bad pitches," Lester said.
Lester was helped by a nice play from left fielder Craig Gentry in the seventh. David Freese hit a leadoff single off the wall and headed toward second as Gentry quickly retrieved the ball and threw him out.
Wilson allowed one run on five hits, struck out three and walked three in 6 1-3 innings.
"The A's do a lot of things well. They pitch well and they have a good bullpen. It was another close game and we didn't get a couple of things done," Scioscia said. "We have to keep moving forward."
Scioscia challenged in the third that Albert Pujols beat out an infield hit rather than recording the third out. The call was overturned in 2 minutes, 4 seconds, and ruled a single.
In the sixth, umpires went to review again on a challenge by A's manager Bob Melvin. He believed Josh Donaldson, who held up briefly at second, was safe at home on a headfirst slide in which he reached to touch the plate with his left hand as catcher Chris Iannetta tried to tag him. The ruling stood after 3:40.
The crew chief then reviewed Sam Fuld's caught stealing in the seventh, per Melvin's request. The ruling was confirmed in 1:05.
Los Angeles 3B Gordon Beckham joined his new team, two days after being acquired by the Angels from the White Sox for a player to be named or cash. He earned a start and batted ninth, going 0 for 3 with a pair of called third strikes.
"Gordon will help in a lot of ways," Scioscia said. "Hopefully he'll give us depth at some key positions."
The Angels optioned RHP Cam Bedrosian to Triple-A Salt Lake.
Angels: The series has shifted game times from 7:05 p.m. to 6:05 and now 5:05 for the series finale. RHP Jered Weaver (13-7, 3.70 ERA) tries again for his first win of 2014 against the A's, going 0-2 with a 7.15 ERA in his first two outings.
Athletics: LHP Scott Kazmir (14-5, 2.73) starts the finale. The A's expect to call up Drew Pomeranz to start Wednesday at Houston and push Friday winner Sonny Gray to Thursday to keep this weekend's rotation the same for a four-game road series next week with the Angels.