After Peter shouts "The containment!" Egon runs out the door and appears to be stepping on thin air to the side of the door frame. The animation models do not match the background plate.
When Slimer presses his back against the door to close it, he smears slime on the door and a little bit of it seems to cover the doorknob, but in the next close-up the doorknob is clean. The animation layer of the slime was only supposed to cover part of the door, not the knob.