Character mistake: As we know throughout this movie series, Michael's middle name is Audrey. However when the doctor is typing Michael's release form, he types his middle initial as "M".
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Suggested correction: Michael's middle name is only mentioned in an additional scene that was added to the television version of the original Halloween. If going by the theatrical versions Michael's middle name was never known, so not really a mistake.
Doctors at a sanatorium would certainly know Michael's middle name. It's been on file for decades.
Character mistake: Phoebe gets hit in the head with a tree branch but she puts her hand on the opposite side she was hit on.