The shamisen is a three-stringed traditional Japanese musical instrument derived from a Chinese instrument, thesanxian. It is played with a plectrum called a bachi.
Possibe Kirby's (Brian Klugman) hair is a nod to Sideshow Bob, "The Simpsons (1989)" character voiced by Kelsey Grammer? And in the closing scene, the hair of Niles' new library assistant possibly a nod to [Sideshow?] Bob's brother Cecil, voiced by David Hyde Pierce?
When interviewing for an internship, Lucius refers to Frasier as a "preeminent psychologist." Frasier is a psychiatrist, not a psychologist.
While in Niles' library, Kirby wears a t-shirt reading "It's not my department."
When Martin (John Mahoney) has to keep calm due to his blood pressure, Daphne (Jane Leeves) forces him to watch romantic movies instead of sports. The movies are "Message in a Bottle (1999)," starring Robin Wright and Kevin Costner, and "Ghost (1990)", starring Demi Moore and Patrick Swayze.