Butler's drive sailed to deep left-center field and bounced straight up at the top of the wall, between the padding and a chain-link railing, then back onto the field. Yankees left fielder Brett Gardner retrieved the ball and threw it back to the infield. Both he and center fielder Curtis Granderson were visibly unhappy on the field after the original ruling.
Initially ruled a home run, the blast led New York manager Joe Girardi out of the visitors' dugout to ask home-plate umpire Chad Fairchild about the call before asking second-base umpire Dana DeMuth, who summoned the other umpires for a review of the play.
Replays showed the ball hitting between the pad at the top of the wall and in front of the railing, which would mean it stayed in play. There is padding on top of the wall and then more padding on top of the chain-link fence railing behind it, which caused confusion as to which part of the fence the ball has to clear to be ruled a home run.
The umpires emerged and confirmed the initial ruling, though, giving Butler his 15th home run this season and putting the Royals up, 4-2. Girardi came back out to ask Fairchild to clarify the rule.
"The replay showed that it hit the top of the wall and hit the wall behind it," Girardi said. "I was under the understanding that it had to go over both walls. Dana DeMuth said that it did not. I think the universal rule, it has to go over both. But if he was incorrect, it'd be a shame to lose a game like that."
Girardi added that Fairchild reiterated what DeMuth said about the ball needing to clear only one wall.
The umpires declined to comment after the game.
"I got told some things," Butler said. "We didn't really have a good angle on that, so [the umpires] have a better view than we do and they have to call New York and get a ruling and look at it. So if they tell me it's a home run, it's a home run."
During the delay, Butler was grabbing his helmet in case he had to go back onto the field. In the visitors' dugout, the Yankees were unhappy, especially closer Mariano Rivera.
"I don't know what happened there, if they don't know the ground rules or what," Rivera said. "But whatever they based their call on, this thing wasn't there.
"Their own guys were looking for that play, and they knew. They knew it was a home run, and if it was a home run, the guy would never have had his helmet on to go to second base."
Kauffman Stadium is one of five ballparks in the Major Leagues without park-specific ground rules, according to Major League Baseball. As a result, if the play hadn't been a home run, it would have been ruled a rule-book double.
The situation was similar to another Butler home run at Kauffman Stadium, a walk-off shot in a 2-0 win over the Angels on June 1 that was also reviewed and ruled a homer. That ball cleared the chain-link fence, however.
Adam Holt is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.