Continuity mistake: In this episode, Steve Urkel appears, depicted as a real person. But Urkel also appeared in a few episodes of Full House, which, within the universe of Step by Step, is a fictional television series.
Visible crew/equipment: When they are taping the family, Carol gets mad and goes upstairs and the camera guy taping her tapes her going up the stairs and when he does, for a second, you can see where the set ends and all the lights.
Revealing mistake: When the band is playing the show at the end, JT is supposed to be playing the bass, but anyone can tell he is not, even when the bass sound is used.
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.