Coincidentally this just happened to me today at a cosplay event. Met couple of TF2 guys who who were just right next to an MLP booth. Best weekend event ever.

This happened to me at least one or two times during college. First was when I discovered a girl in my web design class is also a fan of MLP:FIM by noticing she had an MLP coloring book she was coloring in for fun, and I made the comment to her "I bet Rarity's the easiest to color in." She was pretty cool, occasionally helped me on my school work when I was having trouble. The other was discovering another girl was a fan of Homestuck when she came to class one day wearing a Homestuck godtier hoodie.