bug#17893: 24.4.50; (error "Marker does not point anywhere")

[Drew Adams]

> >> I think we need more data to figure out why the mark isn't set and hence
> >> what to do about it.  A reproducible test case would obviously be best,
> >> but otherwise maybe some indication of what kind of mouse gesture you
> >> did, or what kind of customizations you might have that could interact
> >> with that code and could unset the mark (maybe from
> >> a pre/post-command-hook?).
> > Sorry; If I knew such things I would have mentioned them.  I was not
> > aware of making any mouse gestures or even using the mouse, IIRC.  But
> > I might have moved the mouse somehow; dunno.
> 
> Then maybe it comes from something completely different.
> The clear-temporary-map is called when we "exit" a set-temporary-map, so
> it can also happen with things like C-u, or C-x z, or users of
> `repeat'.  Not sure which ones use `pop-mark', tho.

Maybe.  No idea.  FWIW, there is no `set-temporary-map' in my own code.

Doesn't it seem odd that the backtrace would *start* with
`clear-temporary-map'?  That doesn't look much like it was initiated
by a user action, but perhaps some actions have no reflection in a
backtrace.