StringBuffer should throw NPE when invoked with null however with  -XX:+AggressiveOpts it substitute null instead.

Here is example.
class TSB {
    public static void main (String[] args) {
        while(true) {
            System.out.println(stringmakerBUG());
        }
    }

    public static String stringmakerBUG() {
       try {
           return new StringBuffer(null).toString();
       } catch (NullPointerException e) {
           return "NPE";
       }
    }
}

run:
java  -server -XX:+AggressiveOpts TSB