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: Despite it being demonstrated multiple times earlier in the film that vampires do not have a reflection, a key moment features the undead Mina being revealed in a reflection in water. The filmmakers said they were aware of the discrepancy but ultimately decided the visual was striking enough to bend the rules for.