I think there should be set the pointer to the end as plugin indexing…

… will be unsynced with the methods array.

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=31072

libraries/joomla/event/dispatcher.php

@@ -227,6 +227,7 @@ public function attach($observer)
 
 			$this->_observers[] = $observer;
 			$methods = array_diff(get_class_methods($observer), get_class_methods('JPlugin'));
+			end($this->_observers);
 		}
 
 		$key = key($this->_observers);