"Welcome Back, Kotter" ... Groucho Marx. Here's a nice thing from the NYT Boldface Names about "Welcome Back, Kotter" and ... Groucho Marx:

... RON PALILLO, perhaps best known for his role as ARNOLD HORSHACK in "Welcome Back, KOTTER," [didn't want to talk about various things and] preferred to recount the time GROUCHO MARX arrived on the set to do a guest appearance in 1975 or '76, not long before his death.

"We were his favorite show," Mr. Palillo recalled. "We didn't know what to say, so we all just went up to him and made the famous HARPO raspberry mouth. It really threw him. He just misted over and couldn't speak. They took him out and he never got to do the guest spot."

Of course, "Welcome Back, Kotter" would have reminded Groucho of the Marx Brothers' vaudeville skit "Fun in Hi Skule." And to see all the Kotter boys do the brilliant Harpo mug would have been incredibly endearing and overwhelming for the old man. How sweet! (Boldface doesn't seem to care for the pathos of this scene, being much more involved in finding ways to show that celebrities on the decline are pathetic.)