It's established that clothing and objects worn or held by an alien burn up with them when they die. But when David shoots one of the alien guards, the guard's hat is seen to burn as well, even though it fell off the alien before the burn started.
In the first scene after the main credits, David Vincent is driving a 1967 Ford Galaxie 500 hardtop, but then seconds later it changes into a 1967 Ford Fairlane 500 hardtop.
When aliens were regenerating in their chambers previously a pattern like the x-ray of a normal human being is seen. But when Brother Avery was in the regeneration chamber the "x-ray" pattern looked like two spider like shapes.
When David shoots into the rear window of the car, the pattern of cracks shifts around visibly, revealing that it was added as a post-production optical effect.