Surprising Java behaviour...

Trick.java

import java.util.List;
import java.util.Arrays;
import java.util.Collections;

class Demo {
    static class NotComparable {}

    public static void main(String[] args) {
        List<NotComparable> list = Arrays.asList(new NotComparable(), new NotComparable());

        Collections.sort(list, null);
        // Or the following, which is equivalent:
        // list.sort(null);
    }
}