The Batmobile that Leonard gets to drive is a replica that was built for the TV movie Return to the Batcave: The Misadventures of Adam and Burt (2003).
Howard is seen reading a book entitled "Carl Sagan & Immanuel Velikovsky." This book is actually a defense of Velikofsky's "theories" and an attack on his perhaps most prominent debunker, Carl Sagan. The synopsis on Amazon says, "While brilliantly defending Velikovsky's work, Carl Sagan & Immanuel Velikovsky mercilessly dissects Sagan, the blatantly snide, dishonest and unprofessional 'arguments' and exposes the sordid and unscientific side of 'scientific' controversy."
Title reference: A compatibility questionnaire reveals that Penny and Leonard have different ideas about wanting kids or not.
It is revealed that Leonard has five nephews, all from his unnamed sister.