Continuity mistake: After the sheriff is killed Sam and Dewey leave to go to the hospital and it's daylight. On their drive to the hospital it's dark out. Then when they take Dewey's body out of the hospital and Gayle shows up it's suddenly light again.
Character mistake: Sam tells Mindy at Martha's, "Come on, it's just a movie", in response to her idea the killers are Stab fanatics making a "requel". In diegesis terms, it makes no sense that Sam questions this motive: Dewey states in a previous scene to her, "Your killer's obsessed with the Stab movies, right?" Contextually, she is Billy's daughter, knows his motive was based on horror film obsession; her would-be schoolmate at Woodsboro High (2 grades above), Charlie (Scream 4), was also a Stab fanatic killer. (00:41:00)
Plot hole: Samantha has watched Stab, knows her father's story, and yet does not recognize she is entering Stu Macher's house. The cancelled Stab 3: Return to Woodsboro, seen in Scream 3, confirms Stu's house exterior looks exactly the same from the original Stab, and is being re-used as a prop for that film. (00:01:26)