Chiefs kicker Ryan Succop missed a potential game-winning field goal from 41 yards with four seconds left in regulation but should have had another chance from 36 yards had San Diego been assessed a five-yard penalty.
"On the play, San Diego lined up with seven men on one side of the snapper. This should have been penalized as an illegal formation by the defense," the NFL said in a statement.
"The NFL Rule Book states that 'No more than six Team B players may be on the line of scrimmage on either side of the snapper at the snap.' The rule was adopted this year as a player safety measure."
The Chargers (9-7), who needed a win or tie to reach the playoffs, clinched the AFC's sixth and final postseason berth with a 27-24 overtime win.
But if the correct call was made and Succop connected on the second attempt, the Pittsburgh Steelers (8-8) would have claimed the AFC's final playoff seed based on a tiebreaker.