Shopping online. Easy as can be. Ever actually pay attention to the lists of stuff they recommend for you after you've bought a thing or two?
I was just sifting through my amazon recommend list, and some of it was bang on and others were just not my style.
I've always wondered how one or two book choices or dvds could lead to them thinking your personality is a certain way? And usually the few things they suggest that I like I already have.
Oh, or if you're like me a collector of sorts; you spend so much time and energy and money even tracking down a couple of movies, only to find later after you've gotten it that it/them ended up getting released on a special double/triple/ or four pack. This happens to me all the time. Specially with old vampire movies.