Here is a surefire way to hook into save_post if you need to save stuff. In my case, it is for custom metaboxes.

gistfile1.php

<?php
/**
 * Callback for saving values from metaboxes.
 *
 * @since 1.0.0
 *
 * @param int $post_id The current post ID.
 * @param object $post The current post object.
 */
function tgm_save_meta_boxes( $post_id, $post ) {

    // Bail out if we fail a security check.
    if ( ! isset( $_POST['some-identifier'] ) || ! wp_verify_nonce( $_POST['some-identifier'], 'some-identifier' ) )
        return;

    // Bail out if running an autosave, ajax, cron or revision.
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;
    if ( defined( 'DOING_AJAX' ) && DOING_AJAX )
        return;
    if ( defined( 'DOING_CRON' ) && DOING_CRON )
        return;
    if ( wp_is_post_revision( $post_id ) )
        return;

    // Bail out if the user doesn't have the correct permissions to update the slider.
    if ( ! current_user_can( 'edit_post', $post_id ) )
        return;

    // As you were - it is safe to do stuff now. :-)

}