When Kramer (Michael Richards) runs into Jerry's (Jerry Seinfeld) apartment he leaves the door open behind him. When he goes to leave however, the door is closed and he needs to reopen it.
(at around 6 mins) You can hear that Elaine's voice over is done by someone else when there is a shot of the building before the camera goes into Jerry's apartment.
During the credits, when George begins to run away from The Van Buren Boys, one stage light on a tripod is clearly visible on the right hand side of the set to light up the scene.