Continuity mistake: When Michael first comes home after becoming a vampire he gets some milk out of the refrigerator and when he collapses the whole top of the carton busts open but when his mom comes home and picks it up only one side of the carton is open. (00:37:45 - 00:45:40)
Continuity mistake: In the scene at the beginning of the movie where the flying vampires are chasing the security guard across the boardwalk parking lot, the positions of his arms as he is running changes back and forth between shots. In the close-ups of the guard's face, his hands are up, holding his hat down on his head. In the wide shots, his arms are down at his side. (00:02:45)
Answer: In the film, being invited in isn't a requirement to enter. It's simply a way for the vampire to render you powerless (i.e. the tests the boys performed on Max).