The Bulls forward went to get drug tested, and he didn't come back onto the court when the two teams were brought out of the locker room with 0.5 seconds remaining, according to K.C. Johnson of the Chicago Tribune.
He was instead subbed out on the final possession of the eventual 108-107 Chicago win.
The 76ers failed on their initial attempt to win the game on an alley-oop, but the clock began before they inbounded the ball.
Both teams figured the game was over, but officials brought the squads back onto the floor for a redo of the final play:
Philadelphia couldn't take advantage of its second chance and finished with a one-point loss.
Meanwhile, Bulls head coach Jim Boylen joked after the game that he was happy with the mistake.
"I'm taking our clock guys to dinner," the coach said, per Mike McGraw of the Daily Herald. "That's the way it's supposed to be."
When the officials caught the clock error, Chicago ended up having to play the final 0.5 seconds without one of its best defenders in Porter.
Next time, everyone will have to make sure the game is truly over before heading to the locker room.