Basketball doesn't exactly lend itself to accidentally winding up with too many men on the court. It's not like football, where substitutions occur on virtually every play, or hockey, where lines shift right in the middle of the action. Subs in basketball only come during stoppages and need to be cleared at the scorer's table before they can be made. It is very, very difficult to wind up with six players wearing one uniform on a basketball court.
And yet, the Philadelphia 76ers managed to pull it off. After a Furkan Korkmaz 3-pointer from the corner, Chicago Bulls coach Jim Boylen went absolutely nuts on the sideline. He was justified in doing so. The 76ers played that possession with six players on the floor.
The confusion seems to have stemmed from the previous play. Luke Kornet went to the free-throw line for a technical foul. Somehow, a player managed to make it onto the floor without another coming off. The Sixers had Simmons, Korkmaz, Tobias Harris, Mike Scott, Kyle O'Quinn and Josh Richardson on the floor at the same time. Ironically, the Sixers wound up with only four players on the court, as Harris and O'Quinn both tried to scamper off of the court when they realized what was happening.
The 3-pointer was ultimately wiped off of the board and possession was awarded to the Bulls, no harm, no foul. Still, it will be a long time before you see another one of those calls during an NBA game.