Once the team finally hammered Dan Aykroyd's original draft which involved the Ghostbusters more like SWAT team members who worked in the future into something set in the present day, and not requiring a massive budget, the first day of filming was for the montage that comes about midway through the film. A montage of scenes that mostly take place on the streets of New York, and which the production didn't bother getting filming permits for. Which means that, amongst other things, there's a shot when you can see the cast being chased by an IRL security guard.