Lost in Space had a lot of potential before Johnathan Harris started chewing the scenery.
Lost In Space was on the verge of cancellation before Jonathan Harris started chewing the scenery. It aired opposite Batman beginning in 1966, and the producers felt that it had to bring in campy humor, as well as more color and more Dr. Smith and the robot, in order to compete. It worked, to a point.
To flesh out the above a tad more: the initial plan was to have the Dr. Smith character killed off after the first few episodes; hence the original reason for the
Special Guest Star billing for Harris.
Even before the
Batman series debuted (which actually had a shorter run than
Lost In Space); the decision to not only keep the Dr. Smith character around but to reinvent him as well was already in the pipeline. One needs to remember that the show originally aired at 7:30 on Wednesday evenings; so networks may have had some issues with having the Smith character being too dark & sinister for its air-time back then.
Harris not only played a significant role in reinventing his Smith character but Irwin Allen essentially gave him carte blanche with the role; Harris was allowed to rewrite his character's dialogue.
From what I've read, the series' 1968 cancellation boiled down to money. The special effects used, explosions & all, costed a pretty penny (the coin, not Angela Cartwright).
CBS proposed a budget cap for the show's potential 4th season production & Allen essentially refused to accept the terms of said-cap.
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Back to the topic at hand:
One show that definitely lost its way was
Person of Interest. The show's plotline initially involved only Finch, Reese, Carter & Fosco; but in later seasons, more characters were added and the storylines went all over the place & became more of a challenge to follow. My mother loved the show when it first came out but lost interest in the show in its later seasons due to the changes.
One show that became a victim of a timeslot change was
CSI-Miami. To make room for the then-new rebooted
Hawaii Five-0 in the fall of 2010; CBS moved its long-running
CSI-Miami series from its Monday night 10 PM timeslot to Sunday nights at 10 PM. Unfortunately, such a move coming on the heels of football season meant that all Sunday evening shows were preempted when games were being played/showed. As a result, a preempted 10 PM show wouldn't be aired until well over an hour later at times. By the time football season was over, the damage was already done & the series wound up being cancelled.