Deliberate mistake: After Ash's body is incinerated, the interior walls of the Nostromo appear more metallic and grey than they were before. According to Ridley Scott, this was done deliberately to make the alien's presence appear more menacing.
Deliberate mistake: In the scene where the Punks follow the Warriors into the bathroom, Rembrandt sprays the head punk with his red spray paint. However, the spray coming out of the can is clear, not red.
Deliberate mistake: During the space battle, one of Drax's men gets his back pack blasted. (1:46:37) His entire pack explodes so that his jet pack and life support are destroyed. But as a result of that blast, all the parts from his suit FALL DOWN! Even though there's no gravity.
Deliberate mistake: When the inmates first leave their cells we see at least 2 that have white or light colored shirts on instead of prison blue.