 In only the first few episodes of the series (including the pilot),  during the opening credits there is an extra, unspecified kid in the  Lambert/Foster clan. He can be seen as the family enters the  amusement park and scatters to different areas, during  Peggy Rea (qv)s credit, and another instance with  Susan Somers (qv) later in the sequence. Its suspected that  character of Mark Foster was originally cast as another actor, and  later replaced by Christopher Castile (qv), as evidenced by his  absence from the family rollercoaster ride at the end.