I'm not sure about Killer Clowns - sounds like the story was influenced by Stephen King's novel: IT, and Pennywise the killer demonic clown.
There was also the infamous story of the popular clown paintings of the 1970's, which showed a clown's face with a single tear drop staring out from the frame. It turned out that the artist was in fact a serial killer whose paintings were in fact, the manifestation of a fantasy/self perception in his own mind.
Clowns' are scary, as far as I'm concerned.
I've heard of an UE, who became trapped in the mud at the bottom of a derelict barge he was exploring and photographing in New York. Through a rusted hole to the outside and visibly rising tide, children's party balloons appeared on the surface of the water, and anyone who is familiar with the novel: IT, will no doubt understand the humerous connection and slight feeling of trepidation he had.
He managed to finally escape, but lost his mobile phone, and it serves as a reminder of how dangerous UE can be.