Alien (1979)

Revealing mistake: While Parker is decapitating Ash with the fire extinguisher his T-shirt is not torn, just after he says "Ash is a goddam robot" his T-shirt is now torn and held together with a safety pin. (01:23:20)

Revealing mistake: As Ripley stumbles away from the self-destruct system, the cooling rods are not visible. A few seconds later, in close-up, they can be seen to be above the edge of the compartment they are in. (01:32:10)

Revealing mistake: When Ash attacks Ripley and Parker beats him off, the android gets up again and tackles Parker. Look closely as Parker crawls up against the wall...the body of Ash isn't a body at all...it's half a mannequin that doesn't even have a lower body connected to it, and it is visible when Parker's legs spread and you can see that there is nothing under Ash's torso except a big hole where the lower half should connect. (01:23:45)

Revealing mistake: They use the same shot three times when showing the acid eating throughout the ship. After they try to cut the finger off the face sucker, acid begins to burnt through the decks. When they have gone down 1 deck and look up they see where the acid came through. They then move down 1 more deck and look up again. The acid is just beginning to burn through. Then they look up again and it is the same shot that was used before. The same hanging acid on the right with the drip on the left. After the acid has neutralized, they look up at the hole and it is the same shot again. (00:39:50)

Revealing mistake: Parker had hair. When the alien bit through his head the melon they used did not have hair. Also, the seeds can be seen inside the melon when paused at the right moment. (01:33:35)

Revealing mistake: When Parker shoots the blowtorch at Ash, Ash's face begins to melt off. When the skin melts off we see, in slo mo, that the dummy they used has a sealed mouth. (01:27:25)

Revealing mistake: At the start of the film where we see the cockpit for the first time, you can see the camera shake as it bumps into something whilst it pans towards the seats. The sound of the bump can be heard as well.

Revealing mistake: Mirrors were used to make the corridors appear longer, early in the film when Brett is complaining to Parker about the way they are treated by the company, you can see their reflections in the mirror behind them as they walk down the corridor.

Revealing mistake: When Dallas is in the air shaft he shoots downwards with his flamethrower, you see the flames that he is shooting from the bottom of the ladder, but you can see a flamethrower shooting upwards from the side of the shaft and it also moves in an arc.

Revealing mistake: As Ripley, Lambert and Parker leave the room after decapitating Ash, Parker turns around and incinerates Ash's body. As soon as the shot starts, if you look carefully just before the flame actually touches Ash, you can see Ash's head (mostly the skin below his neck) has already melted before the flame, or even the heat for that matter, touches him. The flamethrower had only been set off a split second before the fire reached Ash and by that point the head has already began melting, so the heat made from the fire could not have melted his head that quickly.