It’s not an error, it’s a notice that something in Codeigniter (core directory) does not match the PHP standards. But this is nothing we’ll patch or fix.
If your webserver displays these messages you should take a look at this: http://www.cyberciti.biz/faq/php-stop-notice-variable-warnings-errorlogs/
Edit the file at the mentioned line. I fixed it by putting the returned value in a variable first and THEN return the value. I don’t know anymore what the code exactly was, but it was something like this: