*- During the introduction scenes of the Nostromo, the stars in the background are maintained even though the spaceship is shown from different angles.
*- When Kane’s helmet is cut off, his yellow leather hood has vanished.
*- While they seem to have no trouble analyzing the chemical makeup of the planet and its atmosphere, they can’t seem to get the radio and video to work at a measly distance of 2 kilometers (as they get to the alien spacecraft). RESPONSE by Richard: This isn’t a blooper its a script flaw. And it isn’t a flaw either. The chemical analaysis of the planet would be performed in a different manner than the the radio transmissions. The chemical makeup can be found either by spectroscopic analysis from the atmosphere or by taking a sample on the way down and feeding it into a Mass Spectrometer. The radio link sucking is easy to explain, there are always pockets of naff reception everywhere look at your mobile phone. The space suits they wear will probably have crap transmitters in them anyway. etc etc [thanks richard!]
*- The facehugger uses its acid to burn through Kane’s mask yet when it is removed there is no visible trace of any acid on Kane’s face.
*- Brett’s cigarette changes size when Ripley confronts him about the way he responds to questions.
*- There is a panoramic view of the ship’s landing area (little lights visible through the wind borne debris) with a ringed planet in the sky following the pull back view of the alien craft. There were no rings during the approach in space.
*- Kane’s shirt changes from being tucked in and out during the chestburster scene.
*- When the alien is popping out of Kane, the camera changes angle and there is a significant amount of blood missing from his shirt. Then when the angle changes again, the blood comes back.
*- From Shannon: There may be a blooper, or just something funny in ALIEN that gets missed. There is a peculiar hole in Parker’s pants you can see while he goes down some narrow stairs as Dallas leads them to intercept the face hugger’s burning acid as it eats through the hull. These things do happen with my own jeans from time to time,,, [thanks Shannon!]
*- During Parker’s fight with Ash, the sleeve on his right shoulder suddenly becomes torn and a safety pin is visible holding it together. In the next shot we see his shirt intact and we see Ash tearing it.
*- When Dallas is crawling around through the bowels of the Nostromo before he is taken by the alien, you can see dolly track lining the floor.
*- Right before Dallas is killed he touches a patch of alien slime on the floor. In the next shot he has more slime on his hand than there was on the floor.
*- The alien’s tail snakes between Lambert’s feet, but it’s not her feet that are shown. Lambert wears cowboy boots; Kane wears joggers.
*- After Ripley fails to cancel the self-destruct mechanism we see a shot of the countdown clock showing about 4 minutes 50 seconds left to destruction. Then there’s a shot of Ripley being frustrated, and then another quick shot of the countdown clock, now showing 10 minutes 39 seconds left to destruction.
*- When trying to abort the countdown, the cancellation instructions Ripley follows with her finger are actually just a French translation of the detonation instructions she followed earlier.
*- Mother’s two 30 second countdowns take 36 and 37 seconds respectively.