At the end of the movie, when the three heroes are in the Head and Shoulders commercial, they pull each pull out a bottle. Harry is clearly holding the bottle with the back label facing forward, rather than the front of the bottle.
Towards the end of the movie, when they first show the party at the College, they show the Jeep is clearly parked and no-one is in it. In the next scene the characters are shown rolling up and parking at the college.
In the opening sequence, when Wayne runs from the meteor about to hit the ground, he isn't wearing his fireman's jacket. When he is shown with his back to the camera looking at the explosion, he is wearing a jacket.
As they are sneaking into the restricted area after dark, Ira and Harry take a belly flop at the razor wire fence. In the next shot they're walking inside the fence and their uniforms have no dirt on them whatsoever.
Toward the end of the film when the cast is discussing using Selenium to kill the aliens, Wayne says "I hate to be a buzz kill here but where are we gonna get 500 gallons of Selenium at two o'clock in the morning?" Then the camera cuts to Danny saying "We can get that". If you look at the window you can see that it's daylight outside. The sun is shining brightly on the bushes.
When the shoplifter is putting on shirts in the stall, she puts on a purple shirt and then a pink one. The camera moves away and back, and the pink shirt is back on the hanger. Then it flips again, and it is on the girl, then back on the hanger again. And when they rescue her from the animal you can see the pink off her shoulder.
During the party at the country club, the blond woman wears red shoes. When she arrived near the green in order to drink Champagne, the shoes are in her hands. After the scene when Barry Cartwright died she came back with the shoes at her feet.
During the "class trip" to the meteor crash site, the gas that the flatworms breath is clinging very low to the ground. Yet, in the very next scene, after they have collected some of the worms and a sample of the atmosphere in a jar, the gas is clearly floating at the top of the jar with the worms having to reach up to breath it.
When the match hits the liquid making the alien grow, one of the legs pushes the coffee cup off the ledge, and you can hear the coffee cup breaking on the floor. If you watch closely the coffee cup breaks as it falls off the ledge.
Near the end, when they go into the nest-cave to fill it with Head and Shoulders, Ira tells Wayne to go out and wait for his signal. Ira and the rest then go into the cave, dragging the fire hose, but as Wayne gets out the fire hose remains still.
When the three hunters shoot the bird in the mall, they walk over to it and it begins to move. All three lower and fire their pump shotguns once, indicating that each now has a spent shell in the chamber. Moments later, when they do the tough-guy move racking the slides in unison, no shell is ejected from any of the shotguns.