Character mistake: When Lisa is taken to the hospital, she tells Kathy that she saw the ghost of Jodie DeFeo. The name of both of the DeFeo's daughters wasn't Jodie. Their names were Dawn and Allison. Jodie was the name of a demonic pig that the Lutz's daughter Missy had befriended.
Suggested correction: While this is true in the original novel, the writers chose to change it to the DeFeo's daughter's name in the movie. Creative license doesn't count as a mistake.
Revealing mistake: Ryan Reynolds is wearing red-eye contacts. You can see the whites of his eyes around the red circle of the contacts in several scenes - most noticeably, right after the home movie he's watching down in the basement melts.
Suggested correction: Not a mistake. Ryan Reynolds actually had bloodshot eyes from being awake for nearly two days of filming. He stated it in an interview for the movie.
Stupidity: Considering the movie takes place in the winter of '75, it seems unlikely that the babysitter would arrive in a halter top and barefeet.
Suggested correction: I agree that this is a "stupidity", but think your use of the words "it seems unlikely" allows for the suggested corrections already given. Perhaps replace "it seems unlikely" to "it isn't rational." Even if the daytime temperature was mild for winter, the temperature would probably drop by around 30° overnight.
Suggested correction: Actually, January of 1975 was very mild, much like the winter in the northeast this year, so it would not be impossible for the babysitter to wear unseasonable clothing.
Suggested correction: Ordinarily I would agree with this observation, however, I believe the girls outfit matches her carefree attitude. She did bong hits in the family bathroom, laid on the boys bed seductively and asked him if he French kissed so dressing comfortably to babysit isn't out of context.
Adding to this, almost every babysitter I had growing up went barefoot or at least shoeless. Even this past new years eve, the young lady who came to watch my kids took her shoes off as soon as she came in the house.
It is a common courtesy to remove shoes when entering someone's home to protect the carpeting (or other flooring) from getting soiled by whatever might be on the bottom of shoes after walking outdoors.
Maybe I'm wrong, but after watching this movie last night, it sounded like the girl was wearing flip-flops when she walked across the bathroom to open the door. Would it be rational for her to have worn sandals too?
Considering she was barefoot for most of the night, yes, she may have worn them for comfort.
