When I've got my car keys in hand (or pocket) and I'm walking past a deep storm-sewer drain, I always make extra certain that I won't drop them, and pause mentally to contemplate my reaction if I were to drop the keys down the drain. Am I the only one who's paranoid about this? Omigod! Google says I'm not! I feel better already. There are two of us. At least I'm brave enough to park my car next to the drain in my parking lot instead of steering clear of it.
Centipedes and millipedes (termed hundred-legged and thousand-legged motherfuckers by my sister) also freak me out. Boy, that was a rough summer the year the millipedes infested the neighborhood. And I'm glad my home is small enough that I can generally reach a shoe to squish a wall-climbing centipede. They move so fast! If one skitters down to the floor, there's little chance of finding it. Gotta get it on the wall. (And then use the Mr. Clean Magic Eraser for the resultant smudge.)
I recently found out that scorpions also alarm me. Here in the now-frigid Midwest, there's little chance of encountering a scorpion outside of captivity. But the other week, I had to check the genus and species of a couple scorpions for a paper I was editing. Seeing a couple pictures and reading a couple paragraphs at Wikipedia? I was undone. Ergo, I will never, ever move to the Southwest.
Plenty of people are afraid of heights, snakes, needles, public speaking, or spiders. What's your most unusual phobia?