Continuity mistake: When Hilary starts crying, Tyrone is wearing an apron. In the following shot he is suddenly not wearing it, but in the next shot it is back on. (00:12:55)
Visible crew/equipment: When Jazz says "oops!" the stage lights are reflected in his glasses.
Did the Earth Move For You? - S2-E1
Other mistake: Viv and Phil are waiting in his office for the power to come back on after the earthquake, but you can see lights on in the building across the street. (00:16:30)
Did the Earth Move For You? - S2-E1
Other mistake: Ashley is now thirteen years old, but she was nine in the previous season.
Suggested correction: It's not uncommon for TV shows to do this, especially with kids. In this exact show they had a baby Nicky who the following season suddenly was like 5 or 6 years old. Other shows like Family Ties and Growing Pains did the same thing. Shows are not always shown in real time when it comes to episodes. They can each take place days, weeks, months or years from previous episodes.
This isn't a valid correction. Just because shows do it or it's common doesn't invalidate the mistake. And while shows don't always have to be shown "in real time", all the other characters would have to make the same jump in age, otherwise it's a mistake.
As Bishop73 said, it's a legitimate mistake because the other characters did not age by four years. There was no time jump. Changing her age does not make sense within the "world" of the show.
