Plot hole: Billy's not tall enough to impale Linnea Quigley on deer antlers.
Suggested correction: The movie blatantly shows Billy lifting her high enough above his head to impale her. It's literally right there in the scene. Additionally Linnea Quigley talks about the making of the scene in the documentary "In Search of Darkness Part II," and talks about the actor living her high above his head over and over while making the scene.
Visible crew/equipment: When Terry's killing the nurse you can see the crew's reflection in the passenger side windows.
Plot hole: When Rachael/Ruby is attacked by Pluto in the rafters, Cass, being blind, is able to hear her cry out. When she does, Cass calls out to her by saying "Ruby." However it was a couple scenes later when Rachael tells everyone that she is really Ruby. Cass is surprised to hear this. Therefore, not knowing that Rachael was really Ruby, she should have called out "Rachael" instead of "Ruby" when she was attacked because she didn't know the truth by then.
Continuity mistake: Towards the end of the movie, when Josh and Holly are trying to escape in the car, a mutant is on the roof and starts melting its hand through the windshield to get at Josh. The mutant is leaning over the windshield with its hand sticking through a hole it melted in the glass. In the next shot as they round a corner, the mutant is suddenly towards the back of the roof of the car, just holding on without attacking the windshield. In the next shot, the hole is gone also from the glass. (01:25:45)
Revealing mistake: The outline of Divine's wig is obvious during the closeup when she's scolding her donkey at the beginning.
Other mistake: When they're walking on the Tri System, you can see the wrinkles in the fabric used to create the foggy background.
Continuity mistake: The social worker takes "the brother" to stay for a while at a female friend's place. Her son, "little Earl" has a bandage on his left knee. He peels the bandage off and sets it on the floor to show "the brother alien" his injury. A medium shot shows there is no bandage on the floor. The "alien" puts his hand on the boy's knee, and heals it. The very next shot the bandage is back on the boy's knee and the scene continues.
Continuity mistake: During the Grand National, the leading horse unseats its rider at the third fence (jockey with pink and black spots), but just a few minutes later the same horse and jockey combination are back up with the pace.
Continuity mistake: When the first flying arrow stabs D'Arnot's butt, it lands diagonally. When the angle changes it's horizontal.
Continuity mistake: Legionnaires are being mowed down left and right by enemy fire in a Nazi ambush. One of them falls into a road construction excavation. He falls on his back, but the close-up of his feet touching the power cable shows them facing down.
Revealing mistake: Near the end, Clint Eastwood picks up a lamp in anger and throws it across the room, but there's no power cord attached to the lamp.
Revealing mistake: When the prince's jeep crashes, he is replaced by a stiff dummy with a black bald head (or maybe a black helmet).
Revealing mistake: When Jake is driving his car to run down Vince, there is a quick shot just before he hits him that is obviously a dummy placed in front of the car.
Factual error: Throughout the entire movie, the SBD Dauntlesses they are working on have the national insignia with the red ball in the center. This was discontinued in May 1942.