One of the things that gets drummed into writers very early (usually through critique groups) is the need to keep point of view (PoV) tightly controlled. This helps to build the link between the writer and the reader and allows the reader to get 'into the skin' of the characters.
I am a big advocate of this. However after years of mercilessly going through my manuscripts looking for PoV quibbles, it is interesting to see how very experienced writers sometimes break the rules with PoV.
There is a real temptation to do this. It is certainly a lot easier to simply write from a character's PoV then to stylishly convey the same thoughts or reactions from another characters PoV.
There is also the issue of economy. Sometimes breaking PoV - in a brief aside - helps to keep the pace and maintain the flow. The trick is flagging to the reader that this is coming so they do not get 'jolted'. One of the easiest ways to do this is to simply insert a break between the paragraphs with a '#' or something similar so that the reader knows that this is a new scene. The other thing that I have seen work very effectively is to switch the PoV at natural points in the narrative where the pace changes - for example at the end of the chapter. The main PoV character may leave the scene before the end of the action, so the last few paragraphs can be from another character's PoV.
There still needs to be some sort of signal. If not after a deliberate break, then the first line should clearly signal the change i.e. 'Darius watched Kelly walk away, his eyes narrowing. The cut on his forearm burned as though he had been splashed with acid.'
I have noticed that many of the older genre classics almost tend toward omniscient, describing the action as though from a distance and only dropping the occaisional paragraph that is clearly from one characters PoV. I recently read 'Duke Elric' by Moorcock. It was hard to say at any given time whose PoV I was really in. I was clearly following Elric's actions, but it was almost as though I was listening through a intermediate narrator (no problems drawing me through the story though:)).
Do you ever break the PoV 'rule'? If so when?