I cannot figure out how to correctly `POST` to the API with curl, so feel free to edit this Q to use curl if you know how.  As it stands, clone http://stackapps.com/q/3950/11299 (or install `M-x package-install RET sx RET` from MELPA) and require `sx-load`.

When the following expressions are evaluated, we receive different responses.

<!-- language: lang-lisp -->

    (sx-method-call "answers"
      :id 7192
      :submethod "upvote"
      :auth 'warn
      :url-method 'post
      :keywords (list (cons 'preview 'true))             ;; NOTE
      :site "emacs")

    =>
    [((question_id . 7189)
      (answer_id . 7192)
      (creation_date . 1420761994)
      (last_edit_date . 1420762419)
      (last_activity_date . 1420762419)
      (score . 4)
      (is_accepted . t)
      (owner
       (link . "http://emacs.stackexchange.com/users/105/drew")
       (display_name . "Drew")
       (profile_image . "https://www.gravatar.com/avatar/32a8e553d85d193ee5ae1533ce6ec158?s=128&d=identicon&r=PG&f=1")
       (user_type . "registered")
       (user_id . 105)
       (reputation . 6689)))]

versus

<!-- language: lang-lisp -->

    (sx-method-call "answers"
      :id 7192
      :submethod "upvote"
      :auth 'warn
      :url-method 'post
      :keywords (list (cons 'preview 'false))            ;; NOTE
      :site "emacs")

    =>
    [((question_id . 7189)
      (answer_id . 7192)
      (creation_date . 1420761994)
      (last_edit_date . 1420762419)
      (last_activity_date . 1420762419)
      (score . 3)
      (is_accepted . t)
      (owner
       (link . "http://emacs.stackexchange.com/users/105/drew")
       (display_name . "Drew")
       (profile_image . "https://www.gravatar.com/avatar/32a8e553d85d193ee5ae1533ce6ec158?s=128&d=identicon&r=PG&f=1")
       (user_type . "registered")
       (user_id . 105)
       (reputation . 6689)))]

Note the difference:

- when `preview=true`, `.score=4`
- when `preview=false`, `.score=3`

However, the POST does succeed; `.score` is updated when you re-retrieve the answer:

<!-- language: lang-lisp -->

    (sx-method-call "answers"
      :id 7192
      :site "emacs")

    =>
    [((question_id . 7189)
      (answer_id . 7192)
      (creation_date . 1420761994)
      (last_edit_date . 1420762419)
      (last_activity_date . 1420762419)
      (score . 4)
      (is_accepted . t)
      (owner
       (link . "http://emacs.stackexchange.com/users/105/drew")
       (display_name . "Drew")
       (profile_image . "https://www.gravatar.com/avatar/32a8e553d85d193ee5ae1533ce6ec158?s=128&d=identicon&r=PG&f=1")
       (user_type . "registered")
       (user_id . 105)
       (reputation . 6699)))]

Note that to reproduce the issue with the Elisp examples provided, you must `sx-authenticate` first.