Say one day you meet a girl and fall in love. Nothing wrong with that. However, on your wedding day, when you meet her father, you remember that he's the guy who failed you in biology in the twelfth grade. Okay, odd, but not inconceivable. But wait, isn't her mother the one who sat next to you at a bus station five years ago and gave you great life advice? And her cousin looks familiar, too. Hold on, it seems you've met her entire family at one point or another throughout your entire life. Subtropes: