Right after Moltar tells Chuck D. that he blew up a porta-potty, "911 IS A JOKE" reads on Moltar's monitor.
This is not a reference to the 9-11 attacks on the World Trade Center, because this episode was produced 3 years prior to the attacks. But, during the attacks, some people were in shock, and thought the attacks were 'a joke'.
Space Ghost: Zorak, you have destroyed an important educational tool.
Space Ghost (to Chuck D.): Now Chuck, my favorite rapper is M.C. Escher.
Rappin' Space Goblin: Well, I'm the Space Goblin, and I'm here to say
That stayin' in school is A-okay.
If you wanna good job, and you wanna do right,
Then stay in school... it's outta sight.
Well, I'm the Space Goblin, and I'm here to say
That if you're crossin' the street, you best look both ways.
Space Ghost: Rappin' Space Goblin, what's the "word" on the street regarding a healthy diet?
Rappin' Space Goblin: We-e-e-e-ell, I'm the Space Goblin, and I'm here to say
That eatin' vegetables is A-okay.
Like - Arrggghh! (gets blasted by Zorak)