One time, in a Latin class I took a year ago, a girl who had already established herself as something of an anime and manga nerd or otaku or whatever, walked past my desk while humming U.N Owen Was Her very distinctly.
At this time, I actually hadn't played a Touhou game, I knew very well what it was and how cool the music was and had planned on playing it but just never got around to it.
I turn to her and say in a perplexed voice "U.N Owen Was Her?" and she responds with a shocked look and asks "How the hell do you know that?", flattering... :V
I respond by saying "I knew you'd be a Touhou fan" based purely on her track record.
She then proceeded to quiz me on the series in general, which I got a fair amount right and a fair amount wrong due to not having played any of the games at the time.
Later on in the week I decided to finally get around to playing Touhou, since it had all but slipped my mind until she hummed that song, the girl basically galvanized me into becoming the Touhou fan that I am today.