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: He places it on the altar just after they believe they have vaporized Gozer with the proton packs. At this point, thinking the ghost to have been destroyed, he sets it down as he believes the job is done and he will not need it.
Jazetopher