Alien
Those who spot some of these errors must have keen eyes! Kane, the main character in the 1979 film Alien, is seen wearing a cloth cap under his helmet. However, there is no cap visible after a facehugger bites him and he is taken to the sickbay, where his helmet is cut off.
Other times throughout the film, the reflections do not match the scenes that they should be presenting. It wasn’t a significant blunder, but it was noticed!