Erick Aybar led off the inning with a high chopper off Dan Otero down the first-base line, where Otero and first baseman Brandon Moss converged on the ball and collided with Aybar about halfway to the base.
Otero had the ball when Aybar ran into him on the infield grass, but home-plate umpire Greg Gibson ruled Moss had obstructed Aybar's path to first. Moss was on the chalk and obstruction is called when a fielder hinders a baserunner within the basepath.
Aybar made a late, quick swerve toward the field in an attempt to avoid Moss on the first-base line but was leveled by Otero.
"There is no way either one of us were obstructing the baseline, because he veered inside to get us, and everyone knows, if you're a hitter, and your baseline is blocked, you veer to the outside," Moss said. "It's just the way you do it. It's a disappointing call, but that happens."
Aybar didn't want to go out of the baseline "because as soon as I do that's an automatic out," he said in Spanish.
"I'm running, and when I picked my head up, I see the pitcher and the first baseman together," Aybar said. "I wanted to move, but they were both on top of me."
Oakland manager Bob Melvin came out of the first-base dugout to argue the call and eventually to notify the umpires the A's were playing the game under protest.
"[The umpires said] he has to have a clear lane to the base," Melvin said. "Well, one, he's way out of the baseline and, two, he went to try to make contact with the fielder. Hopefully it's upheld."
According to crew chief Gerry Davis, the call was a "judgment call" by Gibson, and, under Rule 4.19, a "protest shall never be permitted on judgment decisions by the umpire."
Rule 4.19 also states that "no replay of the game will be ordered unless in the opinion of the League President the violation adversely affected the protesting team's chances of winning the game."
Oakland escaped the ninth inning, but lost the game in the 10th.
"Anytime the leadoff guy gets on, it's huge, especially in the ninth inning of a tied game and that guy's speed, and they play small ball," Otero said. "It makes it even that much more difficult. It makes it a lot harder to get out of the inning. It changes the whole inning."
After John McDonald reached on a bunt single and Efren Navarro advanced the runners with a sacrifice bunt, Otero intentionally walked Gordon Beckham to load the bases and bring in Fernando Abad.
Abad got Kole Calhoun to pop out and Oakland summoned Ryan Cook to face Trout with the bases loaded and two outs. Trout rolled a grounder to third, and Josh Donaldson retired Beckham at second.
Cook pitched the 10th inning as well and gave up Howie Kendrick's walk-off sacrifice fly to right that scored Albert Pujols, giving the Angels a two-game lead in the American League West.