When the police car skids to a halt at the beginning of the movie, there are already skid marks on the pavement from previous takes.
When the abductees awake in the alien spaceship, the tape mark for the position of the first abductee's head can be seen on the floor.
When the aliens arrive on Earth, Harper says, "Los Angeles. It means City of Angels." While 'City of Angels' is a common nickname for Los Angeles, the city name actually means simply "The Angels".