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: According to the SBS website, Brendan's disappearance was never really explained. He just kind of faded out. He was hired as the cute "little" one and as he got older he wasn't as cute. When the Lamberts had the little girl, they got their cuteness factor back and didn't need as many kids around.
Carol and Frank continued to tell people they had seven children throughout the remainder of the show's run, but they never referred to Brendan after season 6.