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..."
Krusty when trying to schedule a new act to compete with Angry Dad ends up having the secretary mistaking his request for an animal be for Susan Anton instead of a lemur.
In Springfield Hospital, Maggie's ribbon is red for a second.
When Homer gets out of the cacti on the 'Trap' scene, he has 2 boils on his neck but just before he approaches the dirty diapers, they disappear.
At one stage, Bart and Milhouse are watching Angry Dad together when Homer runs in and starts choking Bart, and Milhouse jumps off the chair. But soon Milhouse is nowhere to be found in the rest of that scene.
Ralph Wiggum is shown in Bart's classroom when the kids are showing off their "Danger Dog" rip-offs, even though Ralph is a second grader. It's not uncommon for elementary schoolers to spend a portion of the day outside of their "homeroom."
When Homer points the ice cream sandwich at the TV and it gets on the screen, the TV is turned off.