Search box form action changed on user/register page

I am seeing this strange behavior on a production Drupal 5 site. On the home page, for example, the search box (Zen Classic theme) looks like this:

 <form action="/"  accept-charset="UTF-8" method="post" id="search-theme-form">
<div><div id="search" class="container-inline">
    <input type="text" maxlength="128" name="search_theme_form_keys" id="edit-search-theme-form-keys"  size="15" value="" title="Enter the terms you wish to search for." class="form-text" /><input type="submit" name="op" id="edit-submit" value="Search"  class="form-submit" />
  <input type="hidden" name="form_token" id="edit-search-theme-form-form-token" value="27881d0551e2fb0b07bf4534be132882"  />
  <input type="hidden" name="form_id" id="edit-search-theme-form" value="search_theme_form"  />
</div>

However, on the user/register page, the form action has changed.

 <form action="/user/register"  accept-charset="UTF-8" method="post" id="search-theme-form">
<div><div id="search" class="container-inline">

    <input type="text" maxlength="128" name="search_theme_form_keys" id="edit-search-theme-form-keys"  size="15" value="" title="Enter the terms you wish to search for." class="form-text" /><input type="submit" name="op" id="edit-submit" value="Search"  class="form-submit" />
  <input type="hidden" name="form_token" id="edit-search-theme-form-form-token" value="57691367f22e79d3be988cb611d7ce47"  />
  <input type="hidden" name="form_id" id="edit-search-theme-form" value="search_theme_form"  />
</div>

So the search form elements are being returned to the user registration function and the submission process is failing with

The form data has been altered, the session-id field is required.