Mine (late as always) is a bit complicated...mostly because I normally think of myself as pretty brave and level headed in emergencies, and don't have any severe phobias. I was in the military and worked as a paramedic for a while right after uni, and think I can handle stressful situations well.
But my ex loved to watch horror movies, and always made me watch them with her, too. So some very messed up ones (like the Ring, Darkness, the Grudge, Darkness Rising, etc.) really have ****ed me up a bit for life. ;_;
So, much like Spence, with my over-active imagination I always worry about things that I cannot *see*...not really a fear of the dark, but worrying about something being there with me in the dark that I can't be sure is there, but it feels like it is.
So now, whenever I am alone and turn on the lights in a dark room my mind worries about someone being there...and I turn on the lights and see it's a person, but my mind can't understand what I'm seeing. So while I want to turn away, my eyes keep looking, trying to understand what it is. Then, whatever it is turns to look at me... ;_;