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.
Stupidity: After the ammo is removed when the team are under the reactor coolers Vasquez is still on point, even though her machine gun doesn't have a flamethrower capability and her hands are too occupied to be able to use one. Should have been replaced by another team member with an 'active' weapon.
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.