In order for Superman, Wonder Woman and Batman to break into the secret room, they knock it down. Four enemies who were waiting for them in front of the door are crushed by the falling door and blood is splattered, forming a pool of blood in front of the door. During the fight that follows, there are several shots where the knocked over door is shown, but the pool of blood at the front of the door has vanished.
The version of Batman in the universe of this movie is called Kirk Langstrom. In the original DC universe, Kirk Langstrom is actually the alter ego of one of Batman's villains, known as the Man-Bat.