Meemaw wonders why Penny and Leonard know about Sheldon's nickname "Moonpie" (because he's nummy-nummy and she could just eat him up), while Amy doesn't. The reason is that Penny had to search for Sheldon's flash drive in The Terminator Decoupling (2009), and found a letter from Meemaw where she called him Moonpie. She then made him explain the meaning of the term in exchange for her help.
This is the first (and only) on screen appearance of June Squibb as Sheldon's grandmother, otherwise known as Meemaw.
Amy finally learns about the ring that Sheldon produced in The Commitment Determination (2015), and later explained that it was an engagement ring for Amy in The Spock Resonance (2015).
First appearance of Alessandra Torresani as Claire. Alessandra also portrayed Zoey, the very first Cylon ever to be created in Caprica (2009), the prequel series to Battlestar Galactica (2004) (a show that the four guys watch).
When Amy and Meemaw are arguing, Leonard and Penny leave the flat without taking the wine with them. Totally out of character for Penny who is apparently a soak.