Indie-curious attendees of this week's PAX East gaming show in Boston can get a "passport" stamped for every game they check out in the show's indie mega-booth. They can't just stop by the booth. They have to do something.
At the Drinkbox Studios part of the booth, where smart attendees wil check out a game called Guacamelee, you get the stamp if you can play through the first level of Super Mario Bros. with the controller upside down.
I tried it, as you can see in the video here.