Consider the following test case:

import java.util.*;
import java.io.*;

public class Test {
    public static void main(String[] args) throws Exception {
	Properties props = System.getProperties();

	try (OutputStream out = new FileOutputStream("props.xml")) {
	    props.storeToXML(out, "comment", "Vinnie");
	}
    }
}

When run it prints the following to the console:

Warning:  The encoding 'Vinnie' is not supported by the Java runtime.
Warning: encoding "Vinnie" not supported, using UTF-8

It's very impolite for the JDK to print messages to the console, this should be an exception instead.