what is "Fatal error: fatal flex scanner internal error"?

today I have been playing with php5.1.x and on 2 different
machines (completely different files) I got an error rather
like the one below:

Fatal error: fatal flex scanner internal error--end of buffer missed
in /var/www/santos/www/intranet/index.php on line 52

in each file the last 'token' in the file was a comment,
in the index.php mentioned above line 52 is the last line and contains:

// require_once 'end.inc.php';

is there something broken in the engine whereby the last token is not
allowed to be a comment? (I never add the closing '?>' because it
saves a lot of 'header outputted at <bla>' kind of errors -- has that been
declared a bad thing all of a sudden?)

rgds,
Jochem

PS: in the other file that I got the 'fatal flex scanner internal error'
the last line was part of a multiline commenting out some php code as follows:

//*
if (defined('DEBUG') && DEBUG) {
// output some debug info to the message handler...
// MsgHandler::addMsg(new Msg(Msg::Debug, '_GET/_POST (merged)', pre_print(array_merge($_GET, $_POST), true)));

 if (function_exists('apc_cache_info') && ini_get('apc.enabled')) {
     MsgHandler::addMsg(new Msg(Msg::Debug, 'APC Cache Info',
             pre_print(apc_cache_info(), true)
             ));
     MsgHandler::addMsg(new Msg(Msg::Debug, 'APC SMA Info',
             pre_print(apc_sma_info(), true)
             ));
 }
 MsgHandler::addMsg(new Msg(Msg::Debug, '_GET',
         pre_print($_GET, true)
         ));
 MsgHandler::addMsg(new Msg(Msg::Debug, '_POST',
         pre_print($_POST, true)
         ));
 MsgHandler::addMsg(new Msg(Msg::Debug, '_SERVER',
         pre_print($_SERVER, true)
         ));
 MsgHandler::addMsg(new Msg(Msg::Debug, 'Make Filter',
         pre_print(Shop::getMake(), true)." ..."
         ));

}
//*/