But under Rule 5.06(b)(3), both runners on base were entitled to advance a base once Cuthbert entered the dugout, sending Seth Brown home from third.
The rule reads: "Each runner, other than the batter, may without liability to be put out, advance one base when: A fielder, after catching a fly ball, steps or falls into any out-of-play area."
Cuthbert said he wasn't aware of the rule.
"At one point when I was getting close to the ball, I looked down to see where I was at -- if it was going to be a tough play," Cuthbert said. "That's why my first instinct was to hold on to the railing to not fall into the dugout. When the ball was coming down, that's the only shot I had.
"Then the umpire said the runners were going to advance a base. I didn't know about that rule. Every day you learn something in baseball."
The run gave Oakland a 9-7 lead and proved to the be difference in the game after Kansas City scored one run in the bottom of the ninth.
"I knew it right away, but you don't see it very often," A's manager Bob Melvin said. "[Third-base umpire Chris Segal] was right on it. That's a huge play, obviously, to get that second run."
The Associated Press contributed to this report.