Peter is seen in his uniform by mistake at the 16:48 mark.
Gorgar is seen as one of the ghosts released, when he went back to his own world in You Can't Teach an Old Demon New Tricks (1990).
When the Ghostbusters return to the Firehouse, the sign can be seen briefly before Ray points out it is missing.
When Bug is caught in the power surge and Egon says, "Slimer, stop him!", Egon's glasses are sunglasses.
When the Grungy's throw the proton packs out of the window, some parts of the packs appears to phase through part of the trash already laying there.
When Ray asks "Where's our stuff?" he is speaking with Peter's voice.