The most obvious mistake in writing fiction is shifting the POV from the viewpoint character (VPC) to another character in the same scene. Less noticeable POV shifts are from the VPC to an omniscient narrator—the all-seeing, all-knowing invisible intelligence—which is really the author in thin disguise. Writers often fail to see these inadvertent shifts, but a good editor will catch them. Here’s an example:
Braelynn rounded the berm and headed for the bluff, fistfuls of her skirt bunched in her hands to keep her legs free. But before she reached the crags, more horsemen appeared on the cliffs. The lass skidded to a halt.
This viewpoint shift occurs when the writer says “The lass.” In the first two sentences, Braelynn is the VPC. After that an omniscient narrator intrudes, because Braelynn would not think of or refer to herself as “The lass.” Only an omniscient narrator or another character—who shouldn’t have the POV—would. To avoid the POV shift, “The lass” should be changed to “She.” That maintains Braelynn’s viewpoint.
Sometimes an omniscient narrator—that crafty bugger—sneaks in here and there with words that describe something that the VPC cannot, or probably cannot, know. One book I edited said a character wore an Armani suit and Italian-made Moreschi shoes and talked to a Bengali sales clerk. An omni narrator could recognize an Armani suit, Italian-made Moreschi shoes, and a person from Bengal, but can the VPC? I doubt it. In most such instances the author has provided these details.
See what I mean? Writers should review their work carefully to eliminate such unintentional viewpoint shifts. Better yet, get an editor to do this for you.