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: 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 ★