The reason this is an odd one is that the error is being generated when the plugin is called for. The line that this refers to reads something like
public function onPrepareContent( &$article, &$params, $limitstart )
which is the Joomla standard way to call a content plugin of this type. It seems that the place that the plugin is being called from is not supplying the $limitstart variable. Is the plugin code in a com_content article? Can you send me admin access to have a look? I can't see that this is a FAQ Slider problem really but it is intriguing...