Revealing mistake: When Newt is sliding across the grated floor, Bishop reaches out to save her and you can clearly see he's standing in a hole to make him look cut in half. (02:11:40)
Continuity mistake: When Bishop is impaled by the Queen alien he spews android fluid/juice all over his chin. In a subsequent shot his chin is clean and he is spewing fluid again. The following shot his chin is once again covered. (02:23:30)
Revealing mistake: When Ripley first burns the egg plants inside the nest, the Queen screams and moves her head. The steel rod used to hold the Queen's head to the rest of her body is easily visible.
Revealing mistake: Right before Ripley discovers all of the eggs that the Alien Queen has laid, look at Newt's fingers as Ripley is carrying her - It's obviously a fake hand. Newt is in fact a dummy in this scene, and others preceding the chase from the Alien Queen. This dummy was created so that Weaver would not get exhausted carrying a real child around for so many scenes/takes.
Revealing mistake: In the Special Edition of the film, in the scene where the Marines are conducting their initial search of the colony, the shot where Hudson and Vasquez enter the "negative situation" room (where they find the hamster) is obviously sped up. You can tell by how quickly Vasquez seems to move when she comes through the door.
Continuity mistake: Director's cut: When the autoguns were mowing down the army of Aliens coming through the service tunnels, there should have been enough acid blood being sprayed out to melt a massive hole in the tunnel. But the acid has no effect at all on the tunnel walls.
Continuity mistake: There are many holes in the 'squad structure' once the marines arrive at Hadley's Hope. Gorman calls up 1st Squad (comprised of Apone, Vasquez, Crowe and Hudson) to approach the colony first. They do. But, if you look closely, there are other marines present. When they arrive at the door Apone goes straight to the console, Hudson holds back to our left and Crowe rests in the background on the right of the screen. Apone then calls Hudson up to do a bypass. If you look closely as Hudson runs up to the console you will see Frost in the background, he steps up to the left of Crowe (his left) and on the left of the screen, you will see Wierzbowski crouch with his flamethrower, slightly obscured by Vasquez (look between her legs). Then 2nd squad is called up. It is comprised of Hicks, Dietrich, Drake, Frost and Wierzbowski. Watch the shot of Vasquez entering the lock and you will see Frost standing roughly where he was when the 1st Squad ran up. Clearly the original scene had all the marines descend on the North Lock simultaneously. The confusion continues when Gorman, Ripley, Burke and Bishop enter the South Lock. They meet up with Hicks' squad yet Dietrich and Wierzbowski are curiously absent.
Continuity mistake: When remote sentries A and B start firing at the aliens in the tunnel, the display on A shows 500 rounds before they start firing. However, both remotes A and B had been previously test-fired by Vasquez and should show lower counts.
Continuity mistake: In the scene close to the end of the film when Ripley is getting the weapons ready in the 2nd drop ship to rescue Newt, she slams in a new ammo clip into the pulse rifle and the counter reads 95 rounds. Moments later when she is in the elevator descending into the aliens lair the counter is reading 42 rounds, even though she hasn't fired a shot.
Continuity mistake: During the Cargo Loader vs. Queen fight, Ripley grabs the Queen and it closes around her neck. In the next shot it is closed around its head. It changes extremely quickly; therefore it couldn't have changed position.
Continuity mistake: At the beginning of Aliens, when the door is cut through, the markings and lettering on the transparent cover of Ripley's suspended animation pod are different from those that were on it when she was asleep at the end of Alien.
Visible crew/equipment: Throughout the sequence when the female pilot is preparing to drop the ship with Ripley and the marines on down to LV426, you can see a crewmember in her reflective sunglasses.
Visible crew/equipment: In the scene at the colony when a large vehicle approaches the camera, the reflection of someone's face can be seen showing the true scale of the miniature.
Continuity mistake: When the loader is dragged into the airlock, the yellow caution light shatters. The light is fully intact in the next shot.
Continuity mistake: When Ripley and the marines arrive on the planet, it is pouring with rain. But a minute after being inside the buildings, they are all dry.
Revealing mistake: When the loader is dragged into the airlock by the Queen, during the shot where you see the orange light get smashed on the floor (seconds before it actually falls in), you can see two wires attached to the loader leading off screen.
Visible crew/equipment: A wire can be see on the Alien Queen's left leg as she descends from the ship just after ripping Bishop in half.
Visible crew/equipment: When an alien rises out of the water to kidnap Newt, you can see two wires lifting its tail out of the water.
Continuity mistake: As the marines first enter the med labs, Drake at the front, there are some neon tubes hanging from the ceiling that slowly rotate to face the camera. As the angle changes and Drake goes round a corner, the neon tubes rotate to face where the camera previously was again in the background.
Revealing mistake: Briefly after landing on the planet, we see the APC trundle across the ground. The way it moves shows it is a model, every little bump makes it jump up and down. This is a shame, considering that when it is inside the facility and in subsequent shots, it looks very good.
Answer: The company doesn't wait 57 years, they built a colony on the planet over 20 years prior to finding Ripley. The company was apparently unaware of the exact location of the derelict spacecraft. After finding Ripley and obtaining information from her, Burke was able to send the Jorden family to the precise location of the derelict spacecraft.