I have a writing-related question that's bugged me for a while and only now occurred to me to ask.

Third person omniscient I get is where you have a narrator who knows everything and can get in everyone's heads. I've seen examples, I've read descriptions of what it is on the internet, everything is fine.

Except, what's the difference between writing an omniscient point of view and just switching between points of view in a way that's wrong? I can't find an explanation of this (perhaps because people only give examples of what's right, not what's wrong) and I just don't know how to tell whether something's been written in the correct point of view or not.

Is there anyone who can explain it to me?