Mailto links containing apostrophes get broken by this filter. This is because of the call to filter_xss_bad_protocol on line 420 of wysiwyg_filter.pages.inc which does a check_plain() resulting in apostrophes getting converted to '. I'm not sure of the best way to fix this, but for now I've just added:

        if (substr($attrinfo['value'], 0, strpos($attrinfo['value'], ':')) == 'mailto') {
          $attrinfo['value'] = decode_entities($attrinfo['value']);
        }