Do you have some old Joomla site (pre Joomla 1.5.6)? then is very likely you will got - or already have - a site running only halfway, full of errors saying
or
Yeah, you might allready guessed it, the PHP 4.* end of life-cycle is wreaking havoc in Joomla 1.0 sites and some early Joomla 1.5 implementations - or even newer sites, having badly coded third party extensions installed.