I met one of these intrusive shifts last night, so obvious it jolted me out of a doze. I was listening to one of these audio books - very restful on the eyes after hours staring at a screen- and two characters were having a converstion. (It's Iain Banks' 'The Steep Approachto Garbadale'
Although third person, most of the action is from the POV of the young man. Anwyay, he's talking away to his love-object, noticing the sunlight on her copper hair, etc, and there's a sudden switch when she compresses her lips and it says 'She wanted to keep it a secret.' It's obviously quite a significant point, so the reader's attention is attracted to it, but there's also no indication that the young man would know this. It didn't say' It seemed as if she wanted to keep it a secret', although he may have inferred from the compression of the lips. It just stood out as an example of a change in POV. It's a bit frustrating not to be able to pinpoint it as it's on a disk. Maybe it was just ambivalent.
So I agree they can be obtrusive but hard for the writer to spot.
I've just finished a Ruth Rendell tape , 'The Lake of Darkness' which has chapters narrated from a single point of view that shifts between chapter. She does this a lot in her books and it works very well, I think, although sentences like ' Fin parked the car some distance from the house' makes it clear it's third person narrative really.
It's used a lot in crime novels where you can be in the mind of the killer in first-person stream-of-consciousness. I seem to remember 'Silence of the Lambs' was like that, and a reent one I read by Nicci Gerrard. It was chilling to have a chapter about what the police were doing and then one where the killer's mind was one step ahead. It made the killer more 'real' than the detectives, so increased the suspense.
Sheila
<Added>I didn't intend the smiley - it was supposed to be a bracket