404 redirecting on include file

Question

Hello all, I have added a 404 page and included the following in the .htaccess

ErrorDocument 404 http://www.game-plan.net/404.php

For some reason I'm not aware of, when i process forms, it doesnt want to do that, it wants to redirect to a 404 page.

Is this anything common? Something I dont know that maybe someone has come across before? Thanks

Code sample -

<form action='process.php' method='POST'>
            <select name='game'>
            <?php

            $q = $database->allGame();
            while($row=mysql_fetch_assoc($q))
            {
                $u=$row['name'];
                echo "<option value=\"$u\">$u</option>";
            }
            ?>
            </select>
            <input type='hidden' name='type' value='1' />
            <input type='hidden' name='joinrank' />
            <input type='submit' value='Proceed' />
            </form>