Joomla! Documentation™

In 1.5.17 a feature was added to to automatically disable Plugins that triggers an error during initialisation. As a result, some sites have experienced problem behavior.

To remove this feature:

You can disable the auto-disable behavior by changing the helper.php file found in yours libraries/joomla/plugin folder and removing lines 122-136 :

if(!$shutdown_handler_installed)  
{  
// only register the shutdown function if we are capable of checking the errors (reqs PHP 5.2+)  
if (version_compare("5.2", phpversion(), "<="))  
{  
  // you can only register a static method if it is declared static  
  // we can't declare static b/c it breaks on PHP4  
  // therefore we instantiate the helper for this one purpose  
  $pluginHelper = new JPluginHelper;  
  register_shutdown_function(array($pluginHelper, 'shutdown'));  
  }  
  // we may not have installed the handler, but setting this to true  
  // will prevent us from continually running the version compare  
  $shutdown_handler_installed = true;  
}

Thanks to Johan Janssens for sharing this solution.