Instead of Mr. Lorre's usual vanity card, the credits end with a tribute to Leonard Nimoy, who died on 27 February, 2015. The tribute reads:
"The impact you had on our show and on our lives is everlasting."
"The impact you had on our show and on our lives is everlasting."
This is the first time we see Leonard wearing glasses with actual lenses in them (when Sheldon throws a pie at him). Previously in every episode he just wore the frames. So did Bernadette and Amy.
In The Locomotive Manipulation (2014), Bernadette mentioned she lost a $200 bet to Howard that Amy couldn't get Sheldon on the train with her, and Howard said he would be going to the LEGO store for a "big-ass R2-D2." In this episode, Bernadette is doing taxes and asks Howard why he has receipts from the LEGO store, wondering why he spent $200 on a Lego R2-D2.
There actually is a not for profit organization called Mars One. It accepted open applications to anyone in the world interested in being part of a manned expedition to Mars. Their plan is to send an unmanned space craft in 2020 and a manned crew in 2026.
Sheldon says that he wants to buy a turtle because it is an animal he can outrun if it goes berserk; and that for the same reason, he chose Leonard as a roommate. In The Speckerman Recurrence (2011), while running away from a bully, Sheldon indeed outran Leonard.