It's one of those most dreaded phrases...especially when you have to hear it from yourself.
When you meet someone, and you see the signs, you really should just run away. When you ignore those signs (claiming exemption due to your extra-coolness and awareness of said signs), you do so at your own risk. When other people warn you of the things you're already starting to see, you should run faster. At this point even if the negatives are completely overt, you still stick around because you have to prove something to all of those people to whom you defended the person-including yourself. It becomes much more difficult to run.
Then they just out and out disappoint you. You sigh miserably and move on.
And you have to say, "Self, I fuckin' told you so."