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Originally the design for the alien called for it to have pulsing brains in its head (the idea being so we could see the creature 'thinking' while it moved around). The effects department accomplished this by having live maggots placed underneath the transparent dome on the head. Unfortunately the maggots refused to wriggle and move around on command, so the effects dept. doused them with LSD right before the cameras started rolling. The idea was later scrapped. See more...
Alien (1979) - 48 mistakes
Directed by Ridley Scott, starring John Hurt, Sigourney Weaver, Tom Skerritt (add more)
Revealing: In the rapid closeup sequence in which the alien kills Parker (Yaphet Kotto) by biting him right in the face, a freeze-frame viewing indicates that, at the point of impact, a honeydew melon was used to absorb the bite. Now a honeydew melon has a white external rind, and Kotto is a black actor. Why couldn't the SFX guys, at least, have painted the melon to match Kotto's complexion? This production error is, well, in your face.
Continuity: In regards to the self destruct mechanisim of the Nostromo, It is said that the time from the setting to detonation will be 10 minutes. This is also confirmed by the voice of "Mother" who says, right after Ripley initiates the sequence, that the detonation will take place in "T-Minus 10 minutes" However, in the next shot you see one of the monitors which shows a countdown with 10 minutes and an extra 30+ seconds on it.
Continuity: Disappearing Android Fluid- When Ash has gone crazy and is attacking Ripley, his white android juice/sweat/fluid is flowing from his forehead. After he throws Ripley into the room with the nude pictures on the wall, there is a new shot. In it, Ash is walking into the room, but there is no android fluid on his forehead.
Visible crew/equipment: When Kane is struggling on the table shortly before the alien rips through his chest, just before the first patch of blood appears, his T-shirt rides up and something strapped to his chest can be seen. This is some sort of pack containing fake blood that discharges onto his T-shirt. (Best viewed in slow motion.)
Visible crew/equipment: After Ripley fails to shut off the auto destruct sequence, she runs through a corridor filled with gas. As she exits from the gas right before the shot cuts away we see her pass by a full sized light. Beside this light is the clear arm and head of a lighting crew member. (This is acknowledged on the DVD commentary).
Continuity: In the shot of the alien's tail wrapping around the back of Lambert's legs right before it kills her, the floor is different to what it was before, there is water suddenly falling from the ceiling when there wasn't before and her trousers have changed from white to blue. This is because that shot was originally intended to be used when Brett was killed.
Plot hole: When Ripley goes to access the computer Mother, the entry door opens and closes with a hissing noise, a couple of minutes later you see Ash next to Ripley with a smile on his face. How did Ash manage to open the same door without it making the hissing sound? Ash could not have been in the room before Ripley came in - once Ripley has sat down at the console, it rotates on a 90 degree angle, this would have made it impossible for Ash to be hiding behind a desk or a computer console without being seen by Ripley. There are many doors on the ship that make the same hissing sound when the doors open, an obvious example is the infirmary door. And Ash couldn't have disabled the hydraulics of the door, because when Ripley exits the room, the hissing noise is heard again.






