One doesn't automatically dump someone because they like Twilight or because they have toe-thumbs or because they have man-hands. Diagnosing craziness requires a lot of indicators all pointing in the same direction.
If someone is totally undatable, they will have many warning signs. One of my exes 1) was really into gay rights protesting, 2) did not think racist/sexist jokes were funny, 3) was a fine arts student, 4) liked really shitty music, 5) was a frothing-at-the-mouth atheist, 6) did not have a drivers license even though she was 22 years old, 7) would only buy Apple computers, 8) closely followed stuff posted on McSweeney's, 9) was from British Columbia (hippy part of Canada).
You'll notice that all of these point in the same direction. She was lots of fun and knew how to work it but god damn she was crazy.