Factual error: At the very end of the movie, Ray is telling Louis about the Tunguska Blast of 1909, but the famous blast actually occurred in 1908. Given his oft-demonstrated levels of knowledge, he'd know better. (01:37:38)
Factual error: When Mr. Peck comes to the Ghostbusters' building with the police and Con Ed, the NYPD captain is wearing a light blue shirt. All NYPD officers Lieutenant and above wear white shirts. Only patrolmen and sergeants wore the light blue shirt. (01:05:10)
Factual error: When Dana first arrives at her apartment building, you can see a view of her getting ready to cross the street the front door from above, and the demon dog statue's head is in the shot, looking down. If that building is as high as they make it out to be, the distance between the dog, which is on the roof, and the street would be a lot more than it is in the shot. You probably wouldn't even be able to see Dana from that height. (00:16:50)
Factual error: The newspapers in the 'rise to success' montage have all the dates wrong; Usa Today says Tuesday October 8, 1984 (it was a Monday), New York Post says Thursday October 22 (it was a Monday). The USA Today also has a headline "Thin Man dies" which refers to William Powell's death, in March, and the Globe cites "Princess Di expecting again!", which was announced in February (but the fake mag is dated October). (Note; the USA Today details are in the left column and need you to pause, but the others don't - you can also find perfect reproductions of the prop online). (00:39:30)
Factual error: When the Archbishop arrives, Lenny greets him with "Your Eminence," which is a greeting for Cardinals. However, the Archbishop is wearing purple, while a Cardinal would wear red, indicating he was not a Cardinal. New York usually has a Cardinal for an Archbishop, anyway, and the production team likely just assumed the greeting without knowing the color rules. The correct greeting for an Archbishop would be "Your Excellency." In this case, either the greeting or the wardrobe was wrong.
Factual error: Walter Peck in his slimy handshake introduces himself; "I represent the Environmental Protection Agency, the third district." The EPA has no 'districts' (NY's in Region 2). (00:46:15)
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