The simplest theory: the spawn trigger for the “next zombie” was set to activate every time the player’s collision box touched the door’s trigger zone. Because the level designers never linked the spawn to a variable that turns off after a certain number of enemies, it loops forever.
The door swings inward. But the room on the other side is the same hallway you just left.
Instead of the iconic door-opening animation and a loading screen, the player witnesses something truly confusing. The door swings open, but you don’t walk through it. Instead, a zombie—usually one standing directly behind you—seizes the opportunity.