During the supermarket scene, just before the aliens and police shoot at each other near the gas pumps, a cop tries to grab Eric's wheelchair and trips over a car hood.
When Michael tries to pull Eric and his wheelchair into the van while being chased, Michael wears sunglasses when his head is sticking out the van window, but not in the van.
The Mercury Cougar totaled during the car crash scene later drives by, undamaged, during one of the chase scenes.
The close-up version of MAC is less than 2 feet tall. The walking stand-in is almost 4 feet tall.
Crew or equipment visible
When the little girl sucks MAC into the vacuum and gets flung around the room, a track running up the wall, across the ceiling, down the other wall, and across the floor is visible for the entire scene.
When MAC is stretched while holding the chain link fence, then shot into the air, strings are visible holding the puppet.
Near the start of the movie, a cameraman reflected in glass screen at the Army lab.
Old canister-style vacuums do not have a "reversible" motor. Users simply move the hose from one end to the other.
Incorrectly regarded as goofs
When Eric leaves Sears with his mother behind him, the door handles on the store across the street clearly say Sears. The sequence was shot at the Costa Mesa CA store, with a separate merchandise pickup building across the street from the main store.
Eric comes up with the scheme to capture MAC with the vacuum cleaner. He has no prior knowledge that MAC can be sucked in.
When the vacuum pulls Debbie across the ceiling and wall, the girl is an obvious dummy.
When MAC is tap dancing on the counter with the McDonald's crew members behind him, there menu above their heads lists items, but not prices.
Drinks are handed out at McDonalds scene. One falls off the table, and the cup is clearly empty.