One of the many, many people who have taken the court hostage over the years was a woman with a grenade who couldn't distinguish television shows from reality. She was played by Marion Ross, aka Mrs. Cunningham. She lists her many friends which were fictional TV characters and it included "The Fonz".
Actor Existence Failure: Selma Diamond after the second season, and Florence Halop after the third. Marsha Warfield was hired because she was much younger, so this could be prevented from happening a third time.
The Character Died with Him: When Selma Diamond and then Florence Halop died, their respective characters were written as having passed away too.
Colbert Bump: Mel Tormé's guest appearances introduced him to a whole new audience who otherwise would have never heard of him and he developed a following among Generation Xers.
The Danza: Selma Hacker/Selma Diamond, and Florence Kleiner/Florence Halop.
Could also be invoked for Mac Robinson/Charles Robinson.
Averted with Harry Stone played by Harry Anderson, oddly enough. According to series creator and executive producer Reinhold Weege's DVD commentary, the character was called Harry and was a devoted Tormé fan and magician before magician turned actor Harry Anderson auditioned.
Yakov Smirnoff's occasional guest appearances as Yakov Korolenko.
Dawson Casting: Don Cheadle plays a robber in one episode. The character was supposed to be 16, Cheadle was 24 at the time.
Pam Grier was one of the pregnant women trapped in the building by a hurricane.
Dick Butkus was another woman's husband in those same episodes.
Leon, the young shoeshine boy that Harry adopted in season 3, later grew up to be Bumblebee & Dwight.
Al Craven, a sleazy tabloid reporter who hassled the gang a few times during the first two seasons is the titular Bernie.
Estelle Harris, best known as George Costanza's mother on Seinfeld played a hooker that got arrested with Christine's father in season 3.
Bull's girlfriend in one episode was played by the diminutivenote she stands at 4'11, making her the shortest SNL female cast member ever Denny Dillon, from Saturday Night Live (during its notoriously awful sixth seasonnote much like Eddie Murphy and Joe Piscopo, Dillon was one of the few cast members who was a decent actress and genuinely funny during that season despite being given shoddy material to work with, but she ended up getting fired after Dick Ebersol took over) and the HBO sex comedy Dream On.
Michael Richards (before he was on Seinfeld, but after he was on Fridays) as a man who thought he was invisible.
In the Season 2 episode 'Nuts About Harry', James Cromwell and Kevin Peter Hall.
TeriHatcher as Dan's superior's niece whom he's supposed to "keep out of trouble."
Ellen Foley, who played public defender Billie Young in season 2, sang back-up vocals for Meat Loaf. Her best known vocals are on "Paradise By the Dashboard Light" ("I wanna know right now / Will you love me forever?")
Bull's foster son Andy aka Stella from the season 2 episode "Bull Gets a Kid" is played by a young Pamela Adlon (under her maiden name Pamela Segall) best known for voicing boys, like Milo Oblong, Otto Osworth, and, most famously, Bobby Hill.
The Other Darrin: Phil Sanders was played by a different actor (and had a quite different characterization) when he first appears (and is revealed to actually be a wealthy former Wall Street banker who suffered a breakdown).
Reality Subtext: "Flo's Retirement" was a fitting sendoff for Florence Halop, who was dying of breast cancer. And she still managed to finish out the season.
Technology Marches On: Averted, Dan makes use of a cell phone several times, Harry owns several laptop computers And Mac eventually trades in his beloved files for a computer as the series goes on.