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: Because the Head vampire, Max, had fallen in love with Michael's mom. At the end of the movie he said, "Boys need a mother and if I could get Michael and Sam into the family, you would have no choice but to join us."