(all too common at science fiction conventions):
"Hey is that [insert geeky pop culture reference] on your shirt? I love [geeky pop culture reference]. I started collecting [related geeky interest] when I was real little, but then I got into [whatever reference I was wearing] and now I have ALL the [books/DVDs/action figures]. I had a hard time finding the rare one that's out of print, the one that was only released in Japan, but I found it. On eBay. EBay is really good for [books/DVDs/action figures], but you have to know what you're doing. See, [mansplaining the fandom I'm already wearing a shirt for] . . ."
There are plenty of geeky girls who do this too, of course, so it's not always a "pick-up line" thing, but I seem to get this at least once per convention. A lot more if I've got my booth and I'm selling jewelry - I sell a lot of geeky-themed stuff, so everything seems to prompt occasional people to stand there and yack at me. I don't usually mind - it's nice to have someone to talk to! - but I hate when it's a clueless guy who a) seems to believe his knowledge of anime trivia will help him get in my pants, and b) doesn't understand it's not working.