Well, you'd want it to display when you moved to the buffer - If you are
moving out of the overlay in question with C-x o it may not matter, but if
you are moving into a window with a buffer that the point is in one of these
overlays you'd want the action to trigger and display the error message.
The same applies to C-x b.  This was just the use case I had in mind - there
are probably others.

On Sat, Oct 24, 2009 at 10:30 PM, Stefan Monnier
<monnier@iro.umontreal.ca>wrote:

> > The use case I was looking at, as I mentioned earlier, was for
> flymake-like
> > modes to display the actual error messages when point is on an error
> line.
> > This currently looks like it's usually implemented( or is in js2.el, at
> > least ) by having both an overlay and a text property and keeping the two
> in
> > sync.  In this case, I think that doing C-x o to the same buffer should
> run
> > the hooks, and that C-x b should not, which would imply that maybe the
> > buffer is the best place to put them.
>
> Hmmm.... it doesn't seem clear to me why you'd want it for C-x o but not
> for C-x b.  Aren't there other potential use-cases?
>
>
>        Stefan
>