If a program installs the default cookie handler, opens an HttpURLConnection to http://www.google.com, and attempts to call HttpURLConnection.getHeaderFields() on that connection, HttpURLConnection.getHeaderFields() unexpectedly throws the following exception:

Exception in thread "main" java.lang.IllegalArgumentException: Illegal cookie name
at java.net.HttpCookie.<init>(HttpCookie.java:158)
at java.net.HttpCookie.parseInternal(HttpCookie.java:964)
at java.net.HttpCookie.parse(HttpCookie.java:215)
at java.net.HttpCookie.access$100(HttpCookie.java:58)
at java.net.HttpCookie$12.parse(HttpCookie.java:1096)
at sun.net.www.protocol.http.HttpURLConnection.filterHeaderField(HttpURLConnection.java:2605)
at sun.net.www.protocol.http.HttpURLConnection.getFilteredHeaderFields(HttpURLConnection.java:2642)
at sun.net.www.protocol.http.HttpURLConnection.getHeaderFields(HttpURLConnection.java:2686)
at illegalcookienametest.IllegalCookieNameTest.main(IllegalCookieNameTest.java:17)

To reproduce, compile and run the following program (with -DproxyHost=<proxy> -DproxyPort=<port>, if you are behind a proxy):

public class IllegalCookieNameTest {
    public static void main(String[] args) throws IOException {
        CookieHandler.setDefault(new TestCookieHandler());
        URL url = new URL("http://www.google.com/");
        HttpURLConnection c = (HttpURLConnection) url.openConnection();
        c.getHeaderFields();
    }
}

class TestCookieHandler extends CookieHandler {
    @Override
    public Map<String, List<String>> get(URI uri, Map<String, List<String>> requestHeaders) {
        return new HashMap<String, List<String>>();
    }

    @Override
    public void put(URI uri, Map<String, List<String>> responseHeaders) {
    }
}

This is a major problem for JavaFX WebView, see http://javafx-jira.kenai.com/browse/RT-23353