Cancelled on a cliffhanger

[colacentral]

Duckman - Duckman's deceased wife turns up alive; Cornfed says "I can explain," implying he knew about it all along. Cue "to be continued" and cancellation.

The creator recently revealed what the conclusion of the story would be and predictably it was shite.

[Mark Steels Stockbroker]

It's good that Harsh Realm was cancelled, as that opens up a whole vista for fanfic and alternative developments off in to eternity.

(Spam removed)

[NoSleep]

'Soap' spinoff Benson ended on the unresolved cliffhanger of a gubernatorial election result in which the title character was up against his old employer.  According to Wikipedia, Benson would have lost.

You can't mention Benson without mentioning how Soap ended in an almighty crash:

Although Susan Harris had planned for five seasons of Soap, the program was abruptly canceled by ABC after its fourth season. Therefore, the final one-hour episode, which originally aired on April 20, 1981, did not serve as a series finale and instead ended with several unresolved cliffhangers. These involve a suicidal Chester preparing to kill Annie (his wife) & Danny (his son) after catching them in bed together, an irreversibly hypnotized Jodie believing himself to be a 90-year-old Jewish man, Burt preparing to walk into an ambush orchestrated by his political enemies, and Jessica about to be executed by a Communist firing squad. Vlasic Foods pulled their sponsorship of the program shortly after this episode aired and ABC announced that the program was not picked up for its planned fifth season. The official reason given by the network was its declining ratings. However, according to the Museum of Broadcast Communications, Soap "ended under suspicion that resistance from ad agencies may have caused ABC to cancel [it] at that point" because its still controversial content was negatively affecting its relationship with sponsors.

A 1983 episode of Benson mentions Jessica's disappearance, noting the Tate family is seeking to have her declared legally dead. In the episode, Jessica appears as an apparition that only Benson can see or hear and reveals to him that she is not dead, but in a coma somewhere in South America. No other incidents from the final episode of Soap are mentioned.

https://en.wikipedia.org/wiki/Soap_(TV_series)#Later_seasons_and_cancellation

[H-O-W-L]

Dark Angel ended on a cliffhanger that basically changed the entire feckin' world it took place in forever. Cast and crew didn't find out it was cancelled until literally the last moment. Jessica Alba was in the airport about to board a plane to reach the shoot when she got the call.

[NoSleep]

The Expanse would have ended on a similar huge change to its universe had Amazon not decided to take up and continue the series.

[Brundle-Fly]

The recently cancelled TV adaptation of The Mist had a more unexpectedly bleak resolution than the film