Revealing mistake: At the cemetery, a reporter nudges a headstone and it moves, revealing it is fake.
Factual error: There is a scene where the teenage girl says "She lets me drive her car and I don't even have my Learner's Permit yet." The setting is in New Hampshire. New Hampshire does not issue Learner's Permits. NH law is that the driver must be at least 15 1/2 years of age and accompanied by a licensed driver who is at least 25. You can then get a license at 16.
Answer: It was an abandoned boat house, a storage place for marine equipment. Oars, nets, fishing gear. Since it was winter, the hit man knew nobody would be there.