Other mistake: It appears as if Steve is holding 3 $20 bills when he supposedly received $50.
Adventures in Hayleysitting - S8-E6
Other mistake: When Hayley is chasing Steve and his friends in her car, after Steve gets on the bus, Hayley turns around a corner to follow it. When this happens, the license plate on Hayley's car has nothing written on it.
Other mistake: When Francine gives the family gifts, Stan writes "How did you pay for this?" on his board. Francine replies "Well, money, of course" to which Stan writes another sentence on his board. He starts writing without clearing the board, yet when he turns the board back to show the sentence "Is that true, Steve?", the original "How did you pay for this?" isn't visible on the board, despite not clearing it. (00:12:00)
The Devil Wears a Lapel Pin - S11-E7
Other mistake: Francine tells Roger that she's jealous that he's starting college, but in the pilot, she mentions that she met Stan in college.
Chosen answer: It is just the decision of the creators to do so.
MasterOfAll