The scene in which a car is thrown over a cliff was initially intended to be far more spectacular - the car was to fly over the cliff and crash down to the ground. Unfortunately, the sling rope snapped because effects crew members neglected to remove the stoppers from underneath the car's wheels. The result was what is seen in the final film, the car slowly tumbles over the edge and becomes caught on a tree.
There are interesting similarities between The Blob (1958) and this movie. Both have a first victim who is an old geezer living in a cabin and who has a dog. Both movies have a decent cop named Dave who does not believe the young people, and a crabby older cop who seems to have a grudge against young citizens.
This film inspired two serials in Season 25 of Doctor Who named, "The Happiness Patrol" featuring The Candyman (Robot made of candy) and "The Greatest Show in the Galaxy" featuring clowns as villains. Also the serials were originally broadcast on BBC1 in late 1988, the same year this film was released.
The scene where Dave is shooting the clown in the police station the clowns left hand has blood on it from being in Mooney during the dummy scene and in other scenes when they are showing a close up in the shooting there's no blood on his hand
The trivia items below may give away important plot points.
The movie originally ended with the Terenzi brothers as the survivors of the exploding spaceship while Dave perished after killing the giant Klown, but since the executive needed a "upbeat ending" the Chiodo brothers filmed a second ending that is in all the versions where Dave and the Terenzi brothers survive the exploding spaceship by hiding in the "klown kar".
The scene where a Klown rams a car off the road and kills the driver was originally intended to be an opening prologue to the film, and the driver was to be the character Joe Lombardo. For this prologue there was also a scene shot where Joe and his date are kissing on a deserted dark road and after his date rejects him, he angrily drives off alone only to meet his demise with the Klown. However when it was decided to have an opening credit sequence be seen first, the kissing rejection scene ended up on the cutting room floor, and the car crash scene was placed at a later point in the film with the driver not being Joe Lombardo. He ended up being the name of a dead person that Mike and Debbie discover wrapped up in a cotton candy cocoon in the final version.
When Dave leaves with Mike and Debbie to investigate their story about the alien clowns. Mooney says to him "Go ahead Dave, make a dummy out of yourself, but you're not going to make a dummy out of me". It foreshadows what happens to him later on in the film.