You've seen it in your own reading. The story conflict is progressing, and you think it's going to lead you to a really incredible, horrifying, mind-blowing place... and then it never goes there. The scene cuts when you don't want it to and suddenly you're in a different point of view on the outside looking in; or the climax you were expecting turns into something else, which will necessarily be an anti-climax because of the amazing thing you thought you were going to see.
So how do you recognize it - and avoid it - when it happens in your own writing?
There are two ways to approach this: from the personal direction, and from the story direction. From the personal direction, you need to take a look at yourself and identify your own areas of sensitivity - those areas where you're most likely to hold back. For me the main areas are death, sex, and violence. Probably no big surprise. Because I'm very sensitive about those things, it can make them very effective in a story when I use them properly, because the intensity of my reaction to them will come through in the writing. However, my sensitivity means that I'm not the best judge of when I have gone too far, or when I haven't gone far enough.
So, having identified the areas that I need to look out for, I then take a look at the story and try to determine what the story demands. A lot of my sense of this develops while I am writing, and my instincts to follow the principles of the society I'm working with, or the conflict that I'm developing, will take me up to the edge of a sensitive issue - and drop me there wondering what to do next. A big scene of violence isn't something I can just write and plop into my story. It's unnecessary, and gratuitous. But if I'm digging in deep, following the characters, their psychology and their motivations in the context of the restrictions that society puts on them, I can arrive at a point where such a scene grows naturally into the story, and indeed, is demanded by the story so that the feeling of anticlimax doesn't result.
I'm at a point like this right now in For Love, For Power, which is why I'm writing about it.
My story says, "write a scene where Nekantor's gang roughs up prostitutes in a brothel that caters to the noble caste." My admittedly prudish mind says, "eek!"
This is where I go to the next suggestion: get an outside judge. If you can, find someone who is an ideal reader for the piece you're putting together, who knows the vibe of your story and can understand where it's going on its own terms (and not the ones in your head). Again if you can, try to talk it out with them and gauge what level you want to set the scene to - before you've actually attempted to write it.
In my case, I went to Janice Hardy, because she gets this book (and has from the very first partial draft). She pointed out to me a couple of things that are working in my favor: 1. Nekantor is the antagonist, and 2. he doesn't see the world the way I would see it. I think I can use those things to make sure that the moral compass of the story isn't lost when we come into this section, and do a better job of striking it just right. It's not only what happens in the scene that's important, but how the context surrounding it, and the judgments surrounding it, cause it to resonate with the whole.
Of course, I could just try writing something, and wait for the critique stage, for someone to say, "why didn't so-and-so do this?" or "that scene wasn't what I was hoping for." However, I have a terribly hard time hazarding a guess at a scene. I prefer to approach this issue earlier in the process, because my first drafts tend to lose cohesion if I don't work things like this out in advance.
I don't know about you, but I find that the stories I create grow quickly beyond what I expected when I designed them. It's in the nature of the stories. Working in science fiction and fantasy allows a writer to set up principles on an abstract or idealistic level, and then grow them into a world where they can be operationalized on the ground. So how strong are these principles, and how broadly generalized across the culture? That's a question to ask yourself in worldbuilding, as you move toward making the story happen. Once you've decided how far the principles go, however, I think you shouldn't be afraid to discover where they lead you. If you get there and feel unsure, as I often do, ask for outside judgments. When you find exactly the pitch, tone, and approach to make the scene work - and not just work, but sparkle - it will be worth it.