There are actually two competing explanations for this, though it's possible they're both true.

Jonathan Harris ("The Third Man"), the somewhat overacting actor who played the villainous (at first) Dr. Smith, did appear in every episode of "Lost in Space," but he reportedly wasn't meant to. As a result, he was listed as a guest, and they just never changed the credits.

However, he was also the last person to be cast and so, for contractual reasons, he had to be billed last. This apparently seemed like an insult, so he demanded the "special" credit to make himself stand out. (He has admitted this himself in interviews.)

Demanding a special credit to satisfy your own self-opinion is a pretty diva-ish move, but that's very much in keeping with the Dr. Smith character.

According to the first explanation, that character was supposed to be killed off inside the first season, after he fulfilled his instigating role in the story. That role, of course, was to double-cross the Robinson family and sabotage their ship.

That might seem like too much dastardliness for a character to come back from, but Harris managed it. His campy, over-the-top performance reportedly impressed the producers so much that they decided to keep him around full time.

It's also possible that they loved Harris in the role because he embodied it off-screen as well.

Either way, Dr. Smith was by far the most memorable character on the show (perhaps aside from the "danger"-shouting robot), and so he more than earned his "special" designation.

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