I have just found this message
Fatal error: Uncaught OAuthException: An unexpected error has occurred. Please retry your request later. thrown in /home/sloki/user/k0582789/sites/xxxx.xxx/www/wp-content/plugins/recent-facebook-posts/classes/facebook-php-sdk/base_facebook.php on line 1238
fortunately after waiting several minuets, everything goes back to normal, I guess it's a meter of bad connection or problem on facebook side.
Now, I have trace the error message to base_facebook.php on line 1238, and I just wonder, maybe I can modify this 'technically correct' error message into something that more 'humanly' for users to read, something like 'Sorry, bad connection, try later' or 'Houston we got a problem, please reload page'.