Cramped spaces. I'm fat, so this isn't really irrational, but I'm deathly afraid of getting stuck. (Like the time I had to climb into the front seat of my Camry from the back seat because the front doors had frozen shut - I swear sheer panic propelled me into the driver's seat.)
Also, not a fear, but everybody who's ever worked in health care has one thing they cannot deal with, and mine is eyeballs.