It's called author intrusion. The whole point of POV is to be in a character's head. When the character starts thinking or seeing stuff he or she can't possibly see or think, then it's the author intruding his or her view into the story and disrupting the flow. Omniscient is not quite that, in my opinion. If omniscient is the pure POV of the scene, it's not author intrusion. However, if the POV is either first person or third person, and omniscient narrative is mixed in to the scene, THAT'S author intrusion too.
Of course, author intrusion is the whole story! After all, it's you the author writing it. However, to maintain the illusion and immersion into the story, the reader should not be jolted out of the heads of the characters by the author.
Feidb