Revealing mistake: Before the birds attack the helicopter, the close exterior shots reveal that the helicopter is a prop in a set that's not moving a single inch, as opposed to the wide shots of a real flying helicopter.
Other mistake: When they're walking on the Tri System, you can see the wrinkles in the fabric used to create the foggy background.
Other mistake: At the beginning, when Rosie strips down to bathe in the lake, only her skirt comes off, revealing her bare bottom. But not her blouse.
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: 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 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.
Continuity mistake: Whilst the family are on their way to the parents' house, Inoue takes out a sandwich in his mouth to show to a driver in the car beside him. After he takes the sandwich out of his mouth, the sandwich and the hand he is holding it in, rotate 90° in between shots. (00:17:13)
Character mistake: The ref says there's no rules before McClure fights Whiplash but she tries stopping them later.
Continuity mistake: When the coach instructs the guys telling them that their 'pistols don't shoot blanks', the clipboard under his armpit flips around at the cut. (00:06:00)
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.