This is the first time Sheldon (Jim Parsons) says his famous line "I'm not insane. My mother had me tested."
Howard says "So you're just going to sit around and mope while Penny is out with Dr. Apu from the Kwik-E-Mart" to Leonard (Johnny Galecki) when Leonard returns home with dinner. The Simpsons (1989)' character Apu Nahasapeemapetilon actually is a doctor since he holds a PhD in computer science.
Charlie Sheen was a guest star in this episode and at the time was starring in Chuck Lorre's other production Two and a Half Men (2003) as Charlie Harper. Sheen's appearance led to widespread speculation that Two and a Half Men and The Big Bang Theory were set in the same continuity, but later episodes of Two and a Half Men proved otherwise.
Title reference: Sheldon wanted to replace his childhood cat with a griffin, as he thought it would better suit his needs for a companion. He uses the story as a springboard to discuss a replacement for Raj.
Charlie Sheen: As a restaurant patron. At the dinner, after everyone leaves, Raj (Kunal Nayyar) turns to him and says, "Hey buddy, I'm gonna be in People Magazine." Sheen turns around and says "Yeah, call me when you're on the cover." He made the cover of People's Magazine on their 9th March 1987 issue and several times thereafter. He was also named as one of 25 Most Intriguing People in their 2011 issue.