So what is...

[HelenL]

More of a guideline than a rule, I'd say. It can be very effective to use a one-off POV to introduce a scene, particularly one where your main character appears for the first time. George MacDonald Fraser does this in Mr American, for example, which begins with a police officer watching passengers disembark from an ocean liner and then follows the POV of one of those passengers. I've done it a few times, for example Lights, Camera, Blood: Ch. 02 (a taxi driver's POV to establish where the story is set and who the main character is) and The Rivals Ch. 04: The Black Tomb (exposition provided by a bored guard before shifting to one of the two main POV characters).

You can also use it in a scene that involves a main character who's not a POV character. That way you can tell the scene as if the reader is there without burdening yourself with a recurring POV character. In The Dome 03: Over the Edge, for example, POV characters Xero and Ro-Gara have to rescue non-POV character Raurri. I use the POV of one of the captors to describe the situation in the moments before the actual rescue.

It can work, especially at the beginning where you don't really know who the main characters are. Though even there I find it sometimes niggles if a PoV character is dropped cold, and cheer unabashedly if they reappear at a crucial moment. That example sounds like a very strong case.

It's easy to overdo this, of course, but then again it's easy to overdo many things. Otherwise I find it a useful way to keep the story lively and provide the reader with information they otherwise wouldn't get.

*Laughs maniacally in Robert Jordan*

I just finished WoT on audiobook (for the first time, but I've read the dead tree version at least once, several times for the earlier books, and keeping up with his throwaway PoV characters has always annoyed me. Some are great. His habit of killing off the PoV character who just introduced a new situation is effective, but I'm the middle books where the pace drags it's made worse by randomly making someone we already knew a PoV character for a scene or two.