Revealing mistake: When the police car falls into the hole, if you watch the ground that's rising up in front of it, when it finally comes to a stop, you can see how light it is because it bounces, and you can tell that the pieces of the pavement are moved by hydraulics. (01:20:45)
Revealing mistake: Towards the end, when the ground is shaking and breaking up, a police car falls halfway into a hole in the road. When the smoke clears a little you can clearly see two white poles, one on either side of the back left wheel, holding the car in place. (01:21:04)
Revealing mistake: There's a scene where Dana's building is falling apart and stones and stuff are falling to the street below. One of the huge boulders bounces off a wooden police barricade in the bottom left hand corner of the screen, and then off a person. (01:29:14)
Revealing mistake: When the guys are looking down at Stay Puft, as he is about to crush the church, Stay Puft approaches Dana's apartment and you can see a support structure to the right of Stay Puft's right leg. (01:32:00)
Revealing mistake: When the Stay-Puft Marshmallow Man is walking through the city, in several shots you can see outlines of people and buildings through the marshmallow man, who was added in optically. (01:32:05)
Revealing mistake: Near the end of the film when the Ghostbusters take cover in a small corner and come up with the "cross the streams" plan, look closely at Peter's proton pack, you can see that it is a foam "stunt" pack as there is a small sticking out part that keeps squashing against the wall behind him. (01:32:25)
Revealing mistake: In almost all the aerial shots of Dana's apartment block, the traffic on the ground is stationary (NB - this is explained in the SFX documentary on the Special Edition DVD/video - matte paintings were used for those shots)
Revealing mistake: As Dana's apartment crumbles apart to present the temple of Gozer, there is a tight shot of a gargoyle falling off an exterior corner. Carefully lined up with features on the gargoyle's face - but still readily visible - are the black strings that pull the gargoyle face off its mount.
Answer: Ron Jeremy can be seen in the crowd outside the firehouse right after the containment unit is shut off.