I know of a couple of people that if they even see a clown on TV for just a second they start crying or just go into the fetal position. I didn't beleave it myself until I saw it firsthand.
When I was a kid my parents let me watch the original Poltergeist at a young age. I've been scared of them ever since. Stephan Speilberg pretty muched ruined it for clowns and sharks!
I'm not scared of clowns, but I also was allowed to watch Poltergeist when I was young...and the scene with the clown under the bed is probably the scariest thing I remember from growing up.