Tomoe Mami and Kaname Madoka are the only Puella Magi known to have no marks on any of their fingernails. Akemi Homura, Sakura Kyoko, and Miki Sayaka all have marks on their left middle fingers which match the emblem of their respective Soul Gems. Although it would appear strange, it was clarified to be no more than a production error as Homura, Kyoko, and Sayaka do not bear these marks on every single one of their shots in the anime.
Otherwise it would have been an awesome trivia, right? Thanks to again Stilzkin for sending in photos from Anime Expo 2011!