Continuity mistake: Ben pulls Madison into the pool and her hair is straight. When she's dead, her curls magically appear. (01:18:30 - 01:20:00)
Continuity mistake: When Ben drops off Madison at her house, from the passenger side, you can see he has his seat belt on. When he drives away its off.
Plot hole: It's completely unbelievable that a police officer would sit in the back next to Madison instead of the front, or cuff her with her hands in front of her instead of behind her, or sit next to her with their gun in her reach. This would undoubtedly put the officers in a dangerous risk that they would certainly be more observant of.
Revealing mistake: When Ben looks at the picture of Madison, it shows her hands supposedly covering her breasts. But you can see the flesh colored cloth that's protecting her modesty.
Continuity mistake: When Ben is teaching Madison how to swim, the depth of the pool goes from 9ft to 8ft when they move in deeper.