Personally, I have a few twitches. Ever since I was a kid, I made yearly trips to the Chiten Itza, and surrounding, pyramids in Mexico. Since that time, I've developed a very healthy fear of heights. Which is weird as hell, because I love stuff like parkour and try to do it at any chance I get.
Also, flying bugs. If it's in or near my face, my first instinct is to duck or pull back, which I almost always do with the twitchiness of an FPS player. So, butterflies, moths, dragonflies, that kinda thing. If it's near my face, I'll likely have a very miniaturized moment of adrenaline-fueled duck-and-panic.
Movement. Stay with me here, I'm somewhat paranoid. For that reason, I don't usually swim in murky lakes, or take the darker pathes or alleys around me. When I do, I'm on high alert, and usually the first thing I see amiss is also an instant button-press for the flight reflex. I'll start running, usually. You can ask my boss, one time he snuck up on me after work (the entrance we use is actually a side entrance, and I walk the 60 or so yards back to my dorm), and he was parked in my general direction. I heard his shoe scuff when he was just a few feet behind me, and immediately broke into a panicked run. By the time he had gotten to his car a few feet from where he startled me, I was already halfway back to my dorm, still in dead-on full sprint. Also, I've never scaled a brick fence that quickly before...
Mostly, it's just sudden things. I jump easy, and especially moreso when I'm on alert for it.