An illegal choice has been detected

I can't find this in the buglist so I have filed it, but since it is rather obvious it probably already is there somewhere, if someone knows please mark duplicate and link to other issue. This is tested agains a fresh cvs-download.

In "includes/form.inc" there is a check (line 519) elseif (!isset($options[$elements['#value']])) { which I believe fails because the function "form_options_flatten" is wrong. The flatten-function

function form_options_flatten($array, $reset = TRUE) {
  static $return;

  if ($reset) {
    $return = array();
  }

  foreach ($array as $key => $value) {
    if (is_array($value)) {
      form_options_flatten($value, FALSE);
    }
    else {
      $return[$key] = 1;
    }
  }

  return $return;
}

is passed an array of objects

[#options] => Array
        (
            [0] => stdClass Object
                (
                    [option] => Array
                        (
                            [6] => Beef
                        )

                )

            [1] => stdClass Object
                (
                    [option] => Array
                        (
                            [7] => Burger
                        )

                )

but the functions seems expects something else it returns an options-array something like this

Array
(
    [0] => 1
    [1] => 1
    [2] => 1
    [3] => 1
    [4] => 1
)

in this case, if the term-id is higher the 4 this will result in an error.

Can easily be reproduced by creating two categories and then creating some terms. Assign both categories to page, and try to create a new page.