When the power goes out in the family's house, Marty's string of lights on his window remains lit. While some strings of lights are battery-operated, his are later seen plugged into the wall outlet.
In the very first scene after Marty picks up the head from the bowling bag and holds it to his body, the severed head leaves a blood ring in the middle of his gray shirt. However, in the next scene after his mom calls to him, he is wearing the same shirt, with no blood stains whatsoever.