Custom fields: In what order are they saved into the DB?

The SQL query triggered by get_post_custom() is in update_meta_cache() and it looks like this:

$wpdb->get_results( 
    $wpdb->prepare(
        "SELECT $column, meta_key, meta_value FROM $table WHERE $column IN ($id_list)",
        $meta_type
    ), 
    ARRAY_A 
);

So there is no ORDER BY given.

Without ORDER BY the resulting order is almost unpredictable: it depends on the optimizer, used indexes and their order.

The order the fields are saved doesn’t matter here.

You can filter 'query' and add an ORDER BY clause to the query. See wp-includes/wp-db.php – wpdb::query() for details.