Cron Invalid username and/or password, After 4.2.9 to 4.2.20 Joomla Upgrade
This is all due to Joomla changing its password authentication methods on 2.5.18 and above, to fix (not the nicest fix but it works) you need to edit the following.

administrator/components/com_civicrm/civicrm/CRM/Utils/System/Joomla.php

CHANGE TO THIS AROUND LINE 445

  // now check password
  if (strpos($dbPassword, ':') === FALSE) {
    if ($dbPassword != md5($password)) {
      if (!JUserHelper::verifyPassword($password, $dbPassword, $dbId)) {
        return FALSE;
      }
    }
  }
  else {
    list($hash, $salt) = explode(':', $dbPassword);
    $cryptpass = md5($password . $salt);
    if ($hash != $cryptpass) {
      if (!JUserHelper::verifyPassword($password, $dbPassword, $dbId)) {
        return FALSE;
      }
    }
  }