Supposedly, Word of God declares the Clone High Janitor and the Scrubs Janitor to be the same character.
It all makes sense... Glenn acts so nice on Clone High, but the trauma of being fired as well as the death of his son, combined with the trauma of suddenly being in live action, could naturally have shattered the Janitor's mind and left him in the bitter/insane condition that he is in throughout Scrubs. Penny in the door=litter?
He must live in a strange world then.
This theory is validated on the season 8 finale of Scrubs, in which Janitor reveals his name to J.D. as Glenn, the name used by the Clone High character.
Bill Lawrence's wife Christa Miller, who plays Jordan Sullivan, voices Cleopatra.
The background of the credits is a coloured closeup of the center of the golden ratio.
Donald Faison (Turk) plays Joan of Arc's blind father Toots, George Washington Carver, and literally every other black male character.
Zach Braff (JD) makes a cameo in one episode as Paul Revere.
John C. McGinley (Dr. Cox) plays a creepy trucker in one episode.
Sarah Chalke (Elliot) appears in a couple episodes as Marie Antoinette.
The X-Stream Blu promoters? JD, Elliot and Turk.
And from Spin City, Michael J. Fox had a very minor (but fully credited) cameo as Gandhi's Remaining Kidney in the first episode.
MADtv fans should recognise Michael McDonald note (not to be confused with the singer. Michael McDonald was MADtv's longest-running white male cast member, known for his recurring character Stuart, an overgrown kid who lives with his alcoholic mom, played by Mo Collins, and celebrity impressions of Hugh Laurie [as Dr. Gregory House], Chris Hansen, Dr. Drew, Sarah Jessica Parker, Criss Angel, Larry David, Dr. Phil, Maury Povich, Steve Jobs, Jeff Probst, and others. He is to MADtv what Darrell Hammond, Fred Armisen, Phil Hartman, and Bill Hader were to Saturday Night Live, in terms of doing more celebrity impressions than any cast member who has ever been on the show) as Gandhi.