The Janitor's Dad, played by R. Lee Ermey, tells the Janitor to "sound off like [he's] got a pair!" Ermey's most famous portrayal, Gunnery Sergeant Hartman in La chaqueta metálica (1987), often used this line when yelling at the Marine recruits.
The doctor who appears in tight focus right before the end of Elliot and Turk's talk is wearing a name tag that reads 'Dr. Hunter S. Thompson'.
The camera cuts instantly after JD's father (John Ritter) says, "I pooed a little." This is because Ritter ad-libbed this line, causing Zach Braff to immediately burst into laughter.
JD's father (played by John Ritter) is invited by JD to attend a lecture on heart murmurs. John Ritter later died of an aortic dissection (which is a tear in the inner layer of the aorta, the blood vessel that branches off the heart). A possible symptom of aortic dissection is the presence of a heart murmur.
It is revealed in this episode that J.D.'s self-talking / narration stemmed from his childhood. J.D.'s father noticed that and said that he thought J.D. "would have outgrown that by now."