Warning: Parameter 1 to modMainMenuHelper::buildXML() expected to be a reference

At present Joomla! is not certified to work with PHP 5.3.0. If you see this error message on your Joomla website, probably your web host is running PHP 5.3 or recently upgraded to PHP 5.3 (which was my case).

To fix this, go in the helper.php file (/modules/mod_mainmenu/helper.php) and change

function buildXML(&$params)

to

function buildXML($params)

 
Now the error should be gone!