Trivia: When Spengler, Venkman and Stantz are in the basement of the New York Public Library, the bookcase falls over, prompting Venkman to ask Stantz if this has ever happened before. The whole exchange, including the bookcase falling over, was not scripted. It is thought that the bookcase fell because some crewmen accidentally bumped into it, and Bill Murray ad-libbed his lines. Producer/director Ivan Reitman decided to leave it in the movie because it added to the supernatural qualities of the film.
Trivia: The Ghostbusters theme, "composed" by Ray Parker Jr., was directly ripped off from the song "I Want A New Drug" by Huey Lewis and the News. In fact, Huey Lewis sued Ray Parker for intellectual property theft (settled out of court). Premiere Magazine later featured an article in which the film makers admitted to using the song "I Want a New Drug" as temporary background music in many scenes. They said that they made an offer to Huey Lewis to write the main theme for the movie, but Huey Lewis declined. The filmmakers then provided Ray Parker Jr. with finished film footage (including the Huey Lewis song in the background) to aid Ray Parker in writing an original theme song, which apparently he couldn't do.
Continuity mistake: When the Ghostbusters are heading for the building, there's a wide view of New York bathed in a sunset light with a red and blue sky. In further close-up shots, there is a clear sunny light everywhere. (01:15:20)
Character mistake: Meeting Dana at the Lincoln Center fountain, Venkman tells her that Gozer was a deity worshipped in 6,000 BC by Hittites, Mesopotamians and Sumerians. There are no historical accounts of those specific populations dating back to that ancient of a date, or writing of any form for that matter. (00:43:40)