The game of "Rock Paper Scissors Lizard Spock" was invented by a software engineer named Sam Kass, who outlined the rules and hand gestures on his website in 1998. He, however, called it "Rock Paper Scissors Spock Lizard".
When Howard calls in Def-Con 5, Sheldon correctly states that Def-Con 5 is all is well. Def-Con 1 is the highest level of alert where nuclear weapons may be used. The US has only gone to Def-Con 2 once, during the Cuban Missile Crisis and has never gone to Def-Con 1.
When Howard is leaving Stephanie a message the second time, he tells her to claim she's half Jewish "On her mother's side". This is because, unlike in Christianity and Islam which are patrilineal, Judaism is matrilineal.
In the season 1 episode, "The Grasshopper Experiment," Leonard lightly scolded Sheldon for having left with Raj's date, saying friends should not do such things. However, he himself practically does the same thing in this episode, the major difference being that he intended to date Stephanie, while Sheldon had no romantic intentions.
Sheldon is a fan of Saturn 3, Raj likes Deep Space 9, and Leonard proposes watching Babylon 5, since numerically, it's between the two. This devolves into arguing over which is better, and only resolves to somewhat agree that Star Trek 5 is worse than the others.