Thanks Martin!  That's exactly the thing I was missing.
https://www.gnu.org/software/emacs/manual/html_node/elisp/Current-Buffer.html#Current-Buffer


On Tue, Oct 4, 2016 at 9:07 AM martin rudalics <rudalics@gmx.at> wrote:

>  > I'm sorry but I don't understand your statement.  Should there not be an
>  > expectation that (current-buffer)'s return value does not change between
>  > the second and third call in my original snippet?
>
> No.  The primary concern is that the current buffer must not be changed
> by `describe-key' or any other function that uses a temporary buffer to
> display its results.
>
>  > Put another way, under what circumstances should one expect
>  > current-buffer's return value to be stable in user-written elisp?
>
> The current buffer should never be changed by any function, unless it is
> a purpose of that function to change the current buffer.
>
>  > Put a third way, what about describe-keys documentation should lead an
>  > elisp author to understand that current-buffer will not reflect its
> effect
>  > until after a [run-at-time 0]?
>  > (sorry for the barrage of questions; I feel there's a fundamental
> something
>  > I'm missing in play here and trying to see if you know what it is :))
>
> I suppose you missed the following aspect described in section 26.2 of
> the Elisp manual entitled "The Current Buffer":
>
>    When an editing command returns to the editor command loop, Emacs
>    automatically calls `set-buffer' on the buffer shown in the selected
>    window.
>
> [run-at-time 0] clearly runs after that.
>
> martin
>