Arrieta struck out David Freese, bringing up Francisco Cervelli with one out and Marte still at third base. The Cubs were leading, 3-1, but the Pirates were 90 feet from a one-run game.
Cervelli tapped an 0-2 fastball back to Arrieta, who caught Marte off the bag and fired to third baseman Kris Bryant. Marte ran toward home but was forced back to third by Contreras.
Contreras threw the ball to shortstop Javier Baez, covering third base, but Baez dropped it as he attempted to tag Marte. The Pirates' speedy left fielder made one last break for home plate, but Baez recovered and threw to Arrieta, who tagged out Marte.
Pittsburgh challenged the call on the field, objecting that Arrieta blocked the plate in violation of Rule 7.13 regarding home-plate collisions. But the call was confirmed after a replay review. In the end, the out was scored 1-5-2-6-1.
"That's not very typical," Arrieta said.
Cervelli advanced to second amid the confusion, but four pitches later, the inning was over.
Adam Berry has covered the Pirates for MLB.com since 2015. Follow him on Twitter at @adamdberry. This story was not subject to the approval of Major League Baseball or its clubs.