At the chalkboard, Bart is writing out "Dustin Hoffman IS Sam Etic."
Hoffman had appeared in an episode which spoofed "The Graduate" but he didn't have his name appear in the credits. Neither did Michael Jackson when he appeared. I think at the time, making a gues vocal appearance on 'The Simpsons' was looked down upon.
After those appearances, if I remember correctly, Matt Groening put his foot (yellow-skinned and four-toed, no doubt) down and insisted that if celebrities wanted to appear on his show, they'd have to take the credit... and the heat, if any.
"It was the greatest movie I've ever seen in my life!
And you wouldn't believe the celebrities who did cameos.
Dustin Hoffman, Michael Jackson...
Of course they didn't use their real names, but you could tell it was them."