Penny says that Leonard thinks she has seen all Star Wars movies, even though she only saw "the one with the gold robot"; Bernadette then responds that it could be any of them. They are referring to the android C-3PO, who was indeed in all main Star Wars movies, but he didn't always have gold plating: in Star Wars: Episode I - The Phantom Menace (1999), he had no plating at all, and in Star Wars: Episode II - Attack of the Clones (2002), he had silver plating.