The newborn baby in 'Soap'. Boring episode, very cutesy: its proud parents bring it home from the hospital, and put it to bed in its newly decorated nursery full of toys, mobiles hanging from the ceiling, etc. 'Home sweet home' or something twinkling away. Not a funny line or outrageous happening in the whole show, and you're left thinking 'OK that was the worst episode ever, what on earth were they thinking?'.
Until the credits start rolling, and there's a slight creaking sound, and the toys and mobiles all slowly start moving - a bit teddy bear levitates three feet off the ground...
I had nightmares for weeks.
The possessed baby later starts talking in a deep voice, and normal 'Soap' service is resumed, but that episode where it's born is a killer.