Quick zoom-out to omniscient in an otherwise limited story--your thoughts?

I had this situation in one of my WIPs - it was the only way I could convey a certain piece of information, but it was literally the only omniscient part of the whole book. I eventually decided it jarred too heavily, and sacrificed the piece of info (which ended up not being a big deal anyway).

For an example from a much more successful writer than me: I've noticed that the Jack Reacher books (written in very tight first- or third- person POV) occasionally slip into omniscient. I always notice, but the pace is so fast I get swept along before I have the chance to be annoyed lol. Maybe the lesson is, if your book is compelling, readers might notice but forgive you? Honestly though, chances are that the average reader won't notice at all.

How important is it to include that information in that form? Is there another way you could show this that may tip off a reader, but not necessarily the character?

Your example is jarring, I think. I almost expected the exclamation point of a breathless narration break. But that doesn't mean it can't or shouldn't be done. I did this briefly in my (now kaput and so far only) published novel, and when I asked the editor if the break was a problem she said "Wow, paranoid much? This is fine here."

So how to do it well? I wish i knew. In your case, I suspect the knife is revealed at some point. Is there a way to hint at it before, so at the reveal the character and the reader alike say "Of course! I should have seen that!"

Play around with it and see what you come up with.

I’d leave it in for now. The rules don’t exist to be slavishly followed, they’re there for structure. Breaking them is okay if it works. No one here can say whether it works without seeing an example, so all you’re going to get is personal opinions. Which is fine, but doesn’t tell you if your specific piece works or not. If you still like it, leave it in and check it again later. You don’t need to do all your ruthless edits in the second draft.

There is a pretty common style of third person narrative where the bulk of a scene or chapter etc., even most of the story, is in a pretty close limited third--sticking with one pov (maybe even just one pov character for the entire story) at a time and even using their voice in the narrative--but they will start some scenes or chapters with a more distant overview of things. I don't think there is any problem with this as long as the reader isn't confused or left wondering who they are with in the story or where a particular insight or observation comes from.

The issue with suddenly zooming out into a more omniscient narrative mid scene in a story that has been in a limited viewpoint all along is it can give a reader a sort of metaphoric whiplash. But if there's a smooth, logical transition, and if reader expectations have been established early on, this shouldn't be a problem. But if your story has been in a tight limited third throughout, and you suddenly feel the need to back up and explain something the current pov character doesn't know, you might want to ask yourself why this is the case all of a sudden.

It definitely is jarring, but that could work for certain situations. You really have to think about if this is the best or only way to express that detail and if it really needs to be known.