This is an idea i just came up with for the beginning of season 7 of the show. Penny's surname has long been a mystery to us all including the team (or maybe they are hiding it). So, assuming Leonard comes back from his trip to the north sea with stephen hawking, he could on arrival back tell penny "during the trip i was thinking since i've meet you, you haven't told me your surname". There could be some small conversation between the two e.g. a reason for telling them is that "it might be embarrising for her to reveal her surname" but at the end of the episode it gets revealed. Episode 7x1 could be called "The Surname Paradox".