When I was a kid, whenever I had a nightmare my parents would tell me to think about Santa Claus or the Easter Bunny, since they are associated with happiness and fun. Unfortunately, I took that advice, and for a few years was beset by terrifying nightmares wherein a homicidal Santa or a psychopathic Easter Bunny attempted to murder me.
I no longer fear Santa, but am still occasionally faced with an instance where something/someone that is usually considered friendly, or, at worst, utterly innocuous, fills me with dread.
This Christmas, that something/someone is Elmo. (see Part 1 here.)