Slipping out of your point-of-view-character's head can jar a reader right out of the story.
Years ago, I started a book and set it down before I'd finished the first chapter. The precise moment, was when a paragraph began in one character's head, and ended in another character's head. Even worse, those two characters were in different countries, so it wasn't as if it was an omniscient narrator with characters in the same scene.
That point of view shift killed the book for me, and I've never tried anything by that author since.
If you're unfamiliar with the term, a point of view shift is when the author shifts out of the point-of-view-character's head, either into another character, or showing/explaining something that character couldn’t possibly know.