At the beginning of the film, when Max's mother comes in after he calls her, the sound of her opening the door is heard, but Max's door is already a ajar.
When Cassie's mother is talking to her in the kitchen, Cassie gets a bottle of apple juice out of the refrigerator and takes the lid off twice.
After Cassie finishes reading Max 'The Evil Thing', she shuts the door all the way. But when Max comes out screaming a few minutes later, the door is ajar without Max opening it.
Errors in geography
When Sean runs up the stairs to see if Max is in his room, he runs to the left side of the top floor, whereas Max's bedroom is located on the right side.
Properly butchered meat isn't bloody, the red liquid in packages is more of a protein rich liquid that comes from the damaged cells. Therefore, the "blood" they used wouldn't likely attract the bloodsucking head.