In the film's Epilogue, after spending quite a bit of time together trying to escape zombies, Mitch refers to Norman's sister, Courtney, as "Cathy". This may be a goof, or may be an extremely subtle indication of Mitch's indifference to Courtney's attempted seductions. (Any further explanation here would probably be a spoiler.)
When Norman and his classmates are rehearsing for the school play in the beginning of the movie, Norman is holding a script when he begins to read his part. However, after he is interrupted, he is no longer holding a script.
When Norman and company are driving away from the zombies and cause Norman's father to crash, the police officer crashes into the Babcocks' car from the direction the van has just headed rather than where it has just come from.