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.
Crazy Love - S6-E1
Factual error: Carol and Frank leave for a romantic weekend and Dana begins to tutor Rich for a test in English Literature. When Frank and Carol return on Sunday Rich has not only studied for the test but also TAKEN the test and gotten the results back. How is that possible over a weekend?
Continuity mistake: The layout of Frank and Carol's house is inconsistent with the exterior shots. The driveway as seen from their backyard is more shallow and narrow than what is shown in the exterior shots, and the stairway to the left of their front door has a window halfway up, but the exterior shot shows a low section of roof that would completely obscure any window.
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.
Factual error: When they do the opening credits with all the characters, it shows them at an amusement park. It is Six Flags Magic Mountain. At the very end of the opening, they show Six Flags Magic Mountain as if it were next to a beach. You can tell the water was computerized since Six Flags is no where near the ocean.
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.
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.
Torn Between Two Mothers - S5-E14
Continuity mistake: At the end of the episode, Frank is talking to his mother in the kitchen, and she sits down at the table. The green jacket on the chair behind her keeps changing positions between shots.
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.
Video Mania - S3-E6
Plot hole: The plot for this episode is that Mark reluctantly tries out video games for the first time and soon becomes addicted to them. However, the season 1 episode "All in the Family" showed him to already be an avid gamer.
Audio problem: When Cody is playing the blues on his guitar, he is shown to be fretting the higher pitched strings while playing the lower strings open. However only low notes are heard and at least three different chords are heard, which would not be possible if he was playing the strings open.
Continuity mistake: In season 1 Dana turns 16, then in season 3 she is 18, yet they are only celebrating their one year anniversary.
I'll Be Home For Christmas - S4-E12
Continuity mistake: When we see JT arriving at the ski lodge in his car, it appears to be the same car he bought in an earlier season and had to sell for parts.
Character mistake: Mark walks into the house and is surprised to see Cody there, and remarks that he heard he was back. However, just before this, when Mark entered the house, Cody's van - which Mark had to walk by to enter the house - was visible through the open door, so that should have confirmed it for Mark before he saw Cody in person.
Visible crew/equipment: When Cody is saying to Mark "You know it's really good to see you're dating," a crew member steps into the shot on the right side of the screen. (00:08:45)
Visible crew/equipment: When Cody is handing Carol the cashier's check, a camera and a crew member's arm are briefly visible on the right side of the screen.
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.