When someone tells you not to do something, and you know they're right, but you do it anyway to learn the lesson the hard way.

When you know the person your about to date is going to burn you or they are going to be more trouble than they are worth, but you do it anyway for the sex or the experience.

A person who you know you shouldn't have sex with or date because they have issues, but you can't help it, generally because they are hot, or your just that curious, or you like a challenge, or you have shit self esteem too.