Under his pressure suit's helmet, Kane wears a pale hood, but when the helmet is cut off, revealing the Facehugger, the hood has magically disappeared.

During the scene where Ripley makes fun of Brett for being a parrot, the cigarette or match in his mouth noticeably changes size and appearance. Also, at the end of this scene, he puts the second cigarette in his mouth when it was already there in an earlier shot.

Kane's shirt becomes untucked during his convulsions, but it's tucked in again when the Chestburster breaks free.

When Dallas climbs down a ladder in the Nostromo's air ducts just before he's captured, he leaves the Flamethrower and flashlight behind, but at the bottom of the air duct, he has both of these again. It's possible that he reaches over and grabs them before traveling all the way down, but I don't think he'd stop to do that since he's panicking. In fact, I doubt he would've taken off the Flamethrower at all.

One of the doorway lights outside Mother's control room doesn't turn on when Ripley walks up to the door, but in the next shot, the light is on. This happened earlier when Dallas walked up to the door, but the light started to turn on before the next shot.

When the Alien's tail snakes between Lambert's shoes, it's not her shoes that are shown. Lambert wears cowboy boots, not joggers. These belong to Brett.

Mother's two countdowns last for only 36 and 37 seconds. Movies always play with time though, compressing days into minutes, skipping over non-essential action, etc.