What's a combination of the two called? Sloppy, probably.
Actually, a lot of published writers do this, either in different scenes, or sometimes within a scene. There is a continuum of depths in so-called limited third, with some forms having a narrative voice that is distinct from the pov character but that still never reports anything the pov character couldn't see/perceive/think/know, and other forms (deep limited third pov) making the narrative voice as indistinguishable from the authorial voice as possible. The depth of the pov can move some within a story too.
When does it become omniscient? Some people equate omni with head hopping, but it's not really (though an omniscient narrator may choose to tell a reader about more than one character's perceptions within a scene, it needs to be done in a way that's very clear). In my (admittedly still learning) opinion, it's when the narrator is telling you things that the character of focus could not know or perceive.
There are different styles of omniscient too. Some forms hover outside the characters like a camera, only occasionally (if ever) delving into their thoughts or perceptions, other forms will spend a lot of time telling the reader about the internal perceptions of one character in a scene (like in limited third) but may also sometimes step outside that character's perceptions in subtle or not so subtle ways.
We were discussing this in another thread re George RR Martin, who is usually
said to write in limited third. But a careful perusal of his work reveals that he occasionally tells the reader things that his "pov character" (not really the source of the pov, as it turns out) in the chapter would not be aware of, like "Jon did not notice the [insert event] going on behind him," or "While Bran was contemplating this, Hodor carried..."
It's not wrong to do this if the author is aware of it and it is creating the desired effect (and not confusing the reader).
But I'd say that this makes it omniscient and not limited third, though it is a narrative style that has some of the benefits of both.