The last time Sheldon does any form of his old knocking routine.
After Bernadette sends Howard on a guilt trip, he asks her if she is sure she isn't Jewish. Melissa Rauch is Jewish in real life.
When Amy mentions Dr. George Smoot, Sheldon says that they have "a history". Dr. Smoot was previously on The Terminator Decoupling (2009) where Sheldon tried to offer him a research paper, but annoyed Smoot to the point where he asked Sheldon if he was on drugs.
Sheldon implies that Dr. Saul Perlmutter will not support his Nobel Prize nomination. In The Rhinitis Revelation (2011), Sheldon found the doctor undeserving of a Nobel Prize, and that he himself made more accurate diagrams of the expansion of the early universe on the nursery wall with the contents of his diaper. In The Speckerman Recurrence (2011), Sheldon watched Perlmutter's Nobel Prize acceptance ceremony, and accused him of stealing Einstein's cosmological constant.
Leonard says that "Science has a history of difficult people. Look at Newton who was a jerk to Leibniz, and Leibniz who was a jerk to everyone." The rivalry between scientists Isaac Newton (1642-1727) and Gottfried Wilhelm Leibniz (1646-1716) over who developed "infinitesimal calculus" ("the calculus of infinitesimals") was already mentioned in The Maternal Congruence (2009) and The Tesla Recoil (2017). Although Leibniz was the first to publish about it, he was accused of plagiarizing Newton's work, which caused a long-term dispute between them. It is now generally assumed that both men developed their ideas independently from one another.