The television series that is free of continuity errors is truly a rare bird. One way of avoiding them is multi-episode story arcs. Babylon 5 is perhaps the epitome of this concept: the entire plot of the series was mapped out before production began; when it seemed the series was to be cancelled before the plot was completely resolved, changes were made in order to tie up loose ends in a new 'last season.' When the series was renewed, a few new plot lines had to be added, but they did not affect the ultimate story.
We don’t know, of course, but it is possible that the initial five episode series of stories in Lost in Space was a deliberate story arc. It is equally possible that after having produced the most expensive (and unaired) pilot in history up to that time (“No Place to Hide”), Irwin Allen wasn’t about to waste all that footage and made a deliberate decision to reuse as much as possible from the pilot (an astounding 80%) to recoup his investment. This resulted in might call an incidental story arc, but a real arc nonetheless, with an original 40 minutes or so of the pilot being stretched and padded out to over 200 minutes’ worth of story line.
“Invaders from the Fifth Dimension,” written by Shimon Wincelberg may even be a thematic member of this story arc, even though two unrelated episodes intervened between it and the original five. Wincelberg had co-written the pilot with Allen, and some have argued that this episode was originally intended to include the elimination of the Smith character. It is difficult to imagine how one could prove this though, unless the writer’s notes still exist somewhere in a box in an attic, or there is an earlier version of the script in someone’s garage.
“My Friend, Mr. Nobody” and “The Oasis,” the episodes flanking “Invaders” also share a certain thematic unity with the initial five episodes, so one might include them in the initial story arc too. Only “Welcome Stranger” (episode 6) is out of place. Unfortunately, there is little internal evidence to indicate that these episodes were consciously intended to be part of the story arc, or if the similarity is merely fortuitous.
One could argue that “The Keeper,” the series’ only two-part episode, forms a mini-arc of its own. There is even a reuse of one of the Keeper’s traps later in the season (for its original purpose, no less, not just for set dressing). Similarly, “Blast Off into Space” and “Wild Adventure” kicking off season two have a certain logical progression to them, although this is merely logical necessity rather than a necessary plot device.
The only other example of an in-story reference to an earlier episode that I am aware of is in season one’s “Return from Outer Space,” where Smith refers to being punished by Professor Robinson for something he had done previously, ostensibly his behaviour in the preceding episode, “Attack of the Monster Plants.”
Story arcs are useful devices for ensuring the continuity and unity of a series, but they can also make it very difficult to understand individual episodes when see out of order.