When Peter goes to the festival with William Shatner, he takes the family car. Later, when Lois teaches Meg how to drive, they take the same car. The only way they could've done this is if the four of them had gone to the festival together and Peter and William stayed there while Lois and Meg went driving. However, Lois and Meg didn't go to the festival with Peter and William.
During the scenes when Meg and Peter are driving and crash, Meg's hat and shirt are purple, but are pink in every other scene.
The Wonder Years-style narration states that Meg is 15. However, by this point she had tried taking the drivers test once - so she would have to be at least 16.
The seating arrangement of the students preparing to throw paper wads at Meg during the Wonder Years parody changes depending on the camera angle.
When Lois and Peter are in bed, Peter's glasses (dis)appear during shots.
Even without cable they could still watch TV with rabbit ears, which all the TV sets in show has.