Sheldon's Van Dyke is identical to that worn by a parallel-universe Mr. Spock from the 1960s, and contrasts starkly with the barbarian-like - or Klingon-like? - beards and hair on Howard, Leonard and Raj. Also, Howard and Leonard are compelled to curse Sheldon's "Vulcan hearing".
The movie Toy Story is mentioned by Raj just before he, Leonard and Howard approach Mary Cooper's house. Mary Cooper is played by Laurie Metcalf, who did the voice of Andy's mother in Toy Story and its sequels.
When Sheldon's mother claims that her prayer group kept him safe, he criticizes her logic as being "post hoc ergo propter hoc." The Latin translates as "after this, therefore because of this." Sheldon is pointing out that the group's praying for him and his safety cannot be taken as cause and effect.
In both the original broadcast version and the DVD version, Sheldon's mother is clearly drinking a can of Pepsi. In the original broadcast version she later tells Howard "You'll take a cola!" But in the DVD version she says "You'll take a Coke!"
Leonard preserves a snowflake in a solution of polyvinyl acetal resin (also known as Formvar or Vinylec). This is a real thing. The trick was invented in 1940 by a General Electric researcher in Schenectady, NY, who adapted a technique used to cast copies of aluminum crystallization patterns. Before then, snowflake patterns could only be recorded with microphotography.
The trivia item below may give away important plot points.
The episode title may also be a scientific way to define the change, or variation, of the friendship variants in the relationships between Sheldon, Leonard, Raj, Howard and Penny. The fluctuation being Sheldon's feelings towards the others as friends.