Something that's just come to my mind that's inconsistent with the characters' histories...
We all know that Banana Boy (Binky) was held back and had to repeat third grade, right? That makes him at least one year older than Arthur and the rest of the gang, right? Yet, in flashbacks, Binky's always there in their preschool class... unless the preschool basically saw preschoolers of all ages, but I would think usually they would at least split the kids into age groups, wouldn't they? I mean, I don't know, I asn't enrolled in pre-school.
But then again, it's also kind of like in one episode, we see Muffy being transfered to Lakewood in Grade 2, yet again, occasionally in flashbacks, she's in the same preschool class as the others (specifically in one flashback where she mentions spilling juice on herself, and George giving her his before running away).