Drupal 5 doesn't generate unique id attributes for form submit buttons. This means that if there's both a login form and a search box on the same page they will both use the same edit-submit id causing the page to fail validation.

This has been fixed in Drupal 6. You can solve it in Drupal 5 by inserting the following into template.php:

<?php
/**
* Quick fix for the validation error: 'ID "edit-submit" already defined' or edit-name
* There is a solution in d6 core: http://drupal.org/node/111719
*/
function phptemplate_submit($element) {
   static $count_double_id=0;

  $tmp = str_replace('edit-submit', 'edit-submit-'. $count_double_id++, theme('button', $element));
  return str_replace('edit-name', 'edit-name-'. $count_double_id++, $tmp);
}
?>

(Snippet by stevenQ)