MYSQL warning is displayed in ACP and also in ATOM Feed:

Use mysql_free_result to free result sets which were requested using mysql_query()

Appending such error to ATOM Feed causes FEED not to work on most readers.

Also - error is printed although DEBUGs are not enabled in config.

I fixed displaying the error by changing functions.php:

if (defined('DEBUG'))

{ echo '<b>[phpBB Debug] ' . $error_name . '</b>: in file <b>' . $errfile . '</b> on line <b>' . $errline . '</b>: <b>' . $msg_text . '</b><br />' . "\n"; }

But still problem is that MySQL warning is present.