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.
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.
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.
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.