Continuity mistake: During the scene where Ripley is trying to break the window with a chair, there is a scuff mark on it before she hits it. It goes away and comes back. (01:32:15)
Suggested correction: The view from Ripley's side is looking up at the window and from the other side level where there are reflections at the same height (using Ripley as a reference) of the scuffs. There is no way to tell if the scuffs are there or not.
The Director's Cut (and maybe even on the theatrical version) on the 2010 Blu-ray removes the scratches, so I assume they were not supposed to be there initially.
Revealing mistake: When the swarm of aliens drops through the ceiling panels, you can tell that Ripley and the marines don't have magazines in their rifles. It's especially visible when Ripley looks up before Hicks raises the panel and sees the aliens.
Other mistake: They should have had much more than only twenty-six minutes to save Newt. Bishop said that the reactor would blow in four hours after seeing the emergency venting. He said it would take forty minutes to crawl down that pipe, an hour to patch into and align the antenna, thirty minutes to prep the ship, and fifty minutes flight time. 40+60+30+50=180 minutes, gives them an hour spare.
Suggested correction: And how are you so sure everything went smoothly without problems? Any of those things could have taken longer, leaving only 26 minutes.
