Leonard (Johnny Galecki) states that the combined IQ of Sheldon (Jim Parsons) and himself is 360. In a later episode of season 1 Sheldon states his IQ as 187 meaning that Leonard's IQ is 173.
Darth Vader Shampoo and Luke Skywalker Shampoo actually exist, but there isn't a conditioner.
This is the only episode in the series whose title does not refer to a scientific theory or phenomenon.
Raj (Kunal Nayyar)'s hat in this episode has the number 42 on it, likely paying homage to Douglas Adams's book Hitchhikers Guide To The Galaxy where the answer to the ultimate question of life, the universe and everything is 42.
The opening scene at the sperm bank was cut for syndication. Showrunner Chuck Lorre realized that the scene was no longer compatible with the rest of the show, where Sheldon is so protective of his DNA that he would never sell it so carelessly at a sperm bank. It is also established later on that Sheldon has plenty of savings, removing the need for him to earn quick money that way.