Billy places the "Do Not Disturb" sign on the outside doorknob of his motel room. When he is inside the room, the sign is hanging on the inside. The sign is also noticeably missing on the outside when the door is opened into the room later in the scene.
The windshield on Billy's car goes back and forth from broken to not broken.
Billy just got beaten badly by a cop, especially in the face, yet he has barely a scratch the next day and is moving like nothing happened.