 It has long been rumored that Dr. Smith was only supposed to be on  for a few episodes and then get killed off. This was never true,  just a probable scenario imagined by Jonathan Harris (I) (qv), who  was listed as special guest star throughout the run of the series.  This billing occurred because the billing of the other six actors  was already contractually set before the show went into production  as a series. "Special Guest Star" was Harris request and producer  Irwin Allen (I) (qv) eventually agreed.