'forward-page' class should be in array

When other modules try to modify the links by adding an extra class with hook preprocess link, it will break the page because it tries to add an extra array element to a string.

From the api docs for l() function:

If element 'class' is included, it must be an array

Easy fix, in forward_node_view, change

        $links['forward_link'] = array(
          'title'      => $title,
          'href'       => 'forward',
          'html'       => $html,
          'attributes' => array('title' => variable_get('forward_email_title', t('Forward this page to a friend')), 'class' => 'forward-page'),
          'query'      => array('path' => $form_state['values']['path']),
        );

to

        $links['forward_link'] = array(
          'title'      => $title,
          'href'       => 'forward',
          'html'       => $html,
          'attributes' => array('title' => variable_get('forward_email_title', t('Forward this page to a friend')), 'class' => array('forward-page')),
          'query'      => array('path' => $form_state['values']['path']),
        );

Marking as critical because it is wrong and totally breaks your site when using other contrib modules that modify the link classes.
Its just a one line change (line 1226) but I would recommend checking other parts of the module for this.