I have a definitive fear of the dentist. I haven't been since I was 12.
It's not so much the sensations of scraping teeth and the like, but I have an EXTREMELY sensitive gag reflex as well as a phobia of gagging (and throwing up). Having some guy's hand in my mouth just makes me gag about every ten seconds, causing huge huge panic attacks and making it pretty difficult for him to work on my teeth. Gag, remove hand, close mouth, get over it, start all over again...
Srsly.
I'm gonna have to go to the dentist someday and by then I hope I'm so doped up on drugs that I don't care what's done to me!