Not much really scares me, but there are a few little things that freak me out.
Hornets. If I see one, I head straight indooors.
Someone vomiting nearby (if I hear it behind me, I flinch)
Deep black water. I'm not a great swimmer, so the unknown depths have a particular dread factor. Yet I'm nearly fearless in the deep clear teal water of the Caribbean, and love snorkeling.