10 Horror Movies Where NOBODY Guessed The Killer

1981's slasher film Happy Birthday to Me made us all believe that the killer was Ginny Wainright (Melissa Sue Anderson), seemingly driven crazy by a traumatic past event in which none of her school's most popular students attended her birthday party, leading to an accident which caused her mother's death.

But the big twist reveals that, in fact, Ginny isn't the killer at all - it's her "best friend" Ann (Tracey E. Bregman), who has been wearing an elaborate latex mask to disguise herself as Ginny while committing the murders. Talk about commitment to the bit.

Furthermore, Ann reveals that her father had an affair with Ginny's mother, and that the duo are therefore half-sisters. 

And because this isn't a wild enough battery of surprises for an ending, Ginny then kills Ann moments before the police show up, the implication being that Ginny will almost certainly be blamed for all of the deaths throughout the story.