Continuity mistake: When the road cracks and swallows the police car, the distance between the ambulance behind increases and decreases between shots.
Continuity mistake: Stay Puft walks behind a church, which appears or disappears depending on the angle.
Continuity mistake: When the movie is about to end, the Ghostbusters are in front of the building and the pavement starts to crack open. A group of men by the door stumble off-screen to the left. Half a second later, a close-up shows the people repeating all the previous movements.
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