Revealing to your future son-/daughter-in-law that you are, in fact, their true father, thus rendering any marriage prospects impossible, or at best, unlikely.
Most effective when done to a male named Luke.
Person 1: So, you want to marry my daughter, eh?
Person 2: Yes please, sir, I love her very much.
Person 1: Dave, I am your father.
Person 2: NOOOOOOOOOOOOOO!!! Not Vadering!!!