Character mistake: Given how smart the lead is he doesn't seem to know the proper dosage of sleeping meds he put into his mother's coffee. He placed in enough to possibly cause an overdose.
Deadly Friend (1986)
1 character mistake - chronological order
Directed by: Wes Craven
Starring: Kristy Swanson, Anne Twomey, Matthew Labyorteaux, Michael Sharrett
Revealing mistake: When the bully's held up by Sam wires can be faintly seen.
Tom: Hey, she's dead? Hey, what the hell are you doing, you didn't say anything about a dead body, we were supposed to save her life.
Trivia: In a confusing bit of studio meddling, the producers allegedly forced director Wes Craven to reshoot much of the film and make the murders gorier to appeal to the slasher crowd... and then complained that the movie was "too gory" after he delivered exactly what they asked for.