Add javascript confirmation popup on “Move to Trash” link

Put the following in your functions.php:

if (! empty($GLOBALS['pagenow']) && 'post.php' === $GLOBALS['pagenow'])
    add_action('admin_footer', 'trash_click_error');
function trash_click_error() {
    echo <<<JQUERY
<script>
    jQuery(function($) {
        $('#delete-action a').unbind();
        $('#delete-action a').click(function(event) {
            event.preventDefault();
            alert('Error!');
            setTimeout(
                function() {
                    $('#save-action .spinner').hide();
                    $('#publish').removeClass('button-primary-disabled');
                },
                1
            );
        });
    });
</script>
JQUERY;
}