In Springfield Elementary School, the fire alarm tone is a bell. Most schools in the United States do not use bells for fire alarms. They usually use a buzzer or a high-pitched beeping tone, and some modern schools use a voice-evacuation message over a speaker system. However, bells for fire alarms are more commonly found in Canada.
This is Stan doing everything to promote Marvel Comics (which he used to run): the Thing and Spider-Man are Marvel properties, while the Batmobile is from DC Comics (as are, presumably, the comics he is putting to the back of the shelves).
When Better than TV goes into bankruptcy, Milhouse says, "Looks like the bubble burst", referring to the rise and fall of the dot.com industry.
"Danger Dog" lives in large red fire hydrant, a parody of Danger Mouse living in a pillar box.
One of BetterThanTV.com's cartoons is the series "Lou Rawls, Secret Agent", which features an animated Rawls in a trenchcoat trying to escape from a trap, singing "you'll never find... that microfilm of mine..."
While watching When Dinosaurs Go Drunk, where a drunken stegosaurus is depicted stumbling into a tar pit, Homer claims, "Boy, I've been there." In "Bart Gets an Elephant", Homer got trapped in a tar pit.
In Springfield Hospital, Maggie's ribbon is red for a second.