I have to admit, this is one that it's taken me a while to identify, and it's mostly through reading that I was able to see this one.
As you know, I don't write reviews of all the books I and my family read. In fact, out of a typical library run of 50 or so books, I'll review 6 or 7. Why? Those are the books we love or like. The rest -- they're the books that my son or daughter come to me with only negative things to say, or they quit reading them, or they get downright mad about them. Those we return and we never mention here.
We sat at the dinner table one night, and I asked, what was the absolute worst thing an author can do to them, the reader? What was the one thing that turned them off to a book?
It turns out, they hate it when authors eliminate the threat. The danger. The source of conflict.
They want the story conflict to resolve, but they want to know, even at the end, that whatever the source of the conflict was, it's still around. It was, and still is, real (in the story world, at least).
Example: Your MC is trying to save the world from a mad scientist threatening to detonate a nuclear bomb. The race is on in your story to find the bomb and defuse it, but at the end, even after the MC has cut the detonator wires, it's still a bomb. It's still capable of taking out an entire city, or region. It can still end up in the wrong hands, the wiring can be fixed, it could be rewired to go off using a remote, etc. In other words, no matter what your MC did to resolve the story conflict, the bomb can still go ka-boom!
Real Book Example: The Outsiders. At the end, the Greasers and Socs are working toward liking each other, but no one's taken away their guns, knives or fists, and their rivalry has already killed a couple of boys. The threat of gang violence remains.
Example 2: You are mesmerized by a snake charmer's performance. When the charmer is putting away the snake, he inadvertently reveals the snake has no fangs. The thrill, the sense of danger that kept you riveted to the performance, is gone in a poof. Why? The charmer, by de-fanging the cobra, lied to the audience -- you. (He also kills the cobra, which dies from starvation, but that's another matter.) You learn you can't trust anything the snake charmer has said or done.
As I edit my own MS, I'm realizing more and more that even though I'm writing fiction, what I'm doing is establishing a relationship of trust with my reader. My reader trusts that if something is presented as a threat, a source of conflict, at the beginning of the story, then it better still be a threat on the very last page of the book. Otherwise, it's not worth spending 300 pages worrying if the MC is going to make it out OK. The threat needs to remain a threat, even if only on the most basic (think: a defused bomb is still a bomb) level.
Keeping that in mind, the "big reveal" of a story arc should not be that a villain or other character "lied" and the threat that propelled all the action in the book, the conflict generator, was actually a non-threat, from the git-go. That's a quick way to turn off (dare I say, piss off?) your reader. Permanently.
In other words, never de-fang your conflict.
Picture from: https://medium.com/@frederic_38110/the-snake-charmers-secret-bd02f1914073
The author notes, at one point, " You couldn’t believe anything they (snake charmers) told you."