If a fielder, after having made a legal catch, should step or fall into any out-of-play area, the ball is dead and each runner shall advance one base, without liability to be put out, from his last legally touched base at the time the fielder entered such out-of-play area.
The strange ruling gave the Athletics a 9-7 lead, which proved to be incredibly beneficial. The Royals scored a run in their half of the ninth inning on an RBI single from Ryan O’Hearn before ultimately falling by a 9-8 decision.
Cuthbert wasn’t exactly sure what to make of the ruling and admitted that he wasn’t aware that such a rule was in existence. It’s going to be hard to find a more obscure call than the one that took place in the ninth inning of Thursday’s game between the Oakland Athletics and Kansas City Royals.

“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 told MLB.com’s David Adler. “That’s why my first instinct was to hold onto 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.”
While Kansas City is far removed from the playoff race with just a 47-88 record, the call had huge implications for Oakland. The Athletics hold a one-game lead over the Tampa Bay Rays for the second wild card spot in the American League. On the play, as Cuthbert explained, he didn’t have much of a choice in terms of going into the dugout. His momentum was carrying him that way and would’ve risked a potential injury if he tried to stay on the field.
In the top half of the inning, the Athletics had runners at second and third with one out when second baseman Corban Joseph lifted a pop up toward the dugout. Royals third baseman Cheslor Cuthbert went into foul territory and made the catch despite falling out of the field of play. 
While it may not make a ton of sense, Rule 5.06(b)(3)(C) in the MLB rule book does give clarifications to this peculiar situation. 
With Cuthbert stepping into the dugout following the catch, the umpires awarded the Athletics a run on the play and Seth Brown scored due to the ruling.