Error when trying to edit and save a user field
element_validate_number() was added in Drupal 7.6 (or 7.8, I can't quite remember) so it may be that you have an older version of core that doesn't contain that function.

If you can't upgrade your version of core, try adding this into a module file, it should save you the headache:

if (!function_exists('element_validate_number')) {
  function element_validate_number($element, &amp;$form_state) {
    $value = $element['#value'];
    if ($value != '' &amp;&amp; !is_numeric($value)) {
      form_error($element, t('%name must be a number.', array('%name' =&gt; $element['#title'])));
    }
  }
}


In case you're interested element_validate_number() is the replacement for the deprecated _element_validate_number().