Revealing mistake: The hotel where the Ghostbusters conduct their first sorta-successful disinfestation is supposed to be called "Sedgewick Hotel." However when the trio enters the hallway, through the window you can see the sign "Biltmore's Cleaners", and to the left "One Bunker Hill." The real hotel in Los Angeles used for the movie shoot is the Millennium Biltmore, which is in the One Bunker Hill area. (00:29:55)
Revealing mistake: When Ray shoots at Slimer, the proton beam reaches the door in the first shot, and in the next one goes past it; its future path inside the wall is already outlined and burnt to a crisp. (00:33:10)
Revealing mistake: In the very first indirect manifestation of the ghosts (the floating books), you can see on top of the stacks the rigs that allowed the strings to pull the books across [Noticed by Ivan Reitman during the DVD commentary]. (00:01:15)
Revealing mistake: The Ghostbusters' containment unit explodes with a sinister beam that blasts through the rooftop. In the shot with the camera at street level showing New Yorkers running scared, you can see a blatant light 'halo' around one of them as by-product from the optical effect. (01:08:00)
Revealing mistake: When the Ghostbusters are startled by the maid, they instinctively shoot their proton packs at her. You can distinctly see that the energy beam is coming -at - them and is not shot -from - their weapon, and it happens even before Dan Akroyd properly turned around and had any chance to point the 'rifle' part at the target's general direction. (00:31:45)
Revealing mistake: When Dana opens her fridge and a demonic dog opens its jaws to growl "Zuul!", you can literally see the outline of the lightbulb in the hound's throat. (00:20:10)
Revealing mistake: While the Ghostbusters ride to the building, there's a view of New York from the park under a red sky. Watch the buildings on the far left: They have a thick white outline around where the real sky was cut-out to allow for the new one to be put in. (01:18:35)
Revealing mistake: You wouldn't want to sleep inside the Sedgewick hotel, and not because of its ongoing infestation at the 12th floor; a small food cart rolls into a wall and that alone makes it wobble. (00:33:20)
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