Deliberate mistake: Lily was an infant for all of season 5, but beginning with this season, she is around 5 years old. The show didn't jump forward in time, because JT states in a season 5 episode that he is 19, and in the episode this season where he and Rich go to Mexico, he states that he is almost 20, as well as Al and Mark are both still in high school.
Deliberate mistake: When Frank tosses the cards in the end they all hit the camera very obviously. When the episode first aired the last scene was in 3D, which is why he tosses them directly at the camera like that.
Answer: From what I've read, the kid in question was the original actor cast to play Mark and was intended be closer in age to Brendan. For whatever reason, they recast the role with Christopher Castile and didn't bother removing the original actor from the beginning of the opening credits. I guess they figured people would just assume the kid was one of Brendan's friends.
Phaneron ★