Ketchup on eggs.
If one of those first dates is brunch, and your new friend reaches for the ketchup to put on her eggs, RED FLAG! I realize this may seem arbitrary or fussy. Or perhaps you think I'm making a class judgment here. Well, maybe I am! What's wrong with that? All I know is: Nothing good ever comes of ketchup on eggs. And it's really gross.
Rudeness to waiters.
And taxi drivers, and any-one else in a service job. I shouldn't even have to explain why this is a dealbreaker. Just remember that it is.
Scary divorce stories.
It's amazing how much a new prospect will tell you about her life on a first or second date—much more than she knows she's saying. The question is: Do you hear it? If she launches into the story of her messy divorce, is her ex the villain in every respect? To me, that's a red flag right there. Anyone who's emotionally grounded should be able to see that two people, not one, contributed to a divorce.
A deep attachment to disturbing pets.
A golden retriever is fine, and cats are all right if they don't do much. But I'm still haunted by the memory of an ancient, hairless dachshund that would manage to jump up on the bed during inopportune moments and bay. Not until the dog-owner chose to disengage herself from me and comfort the dog instead did I know that this was trouble.