One would think that the following program
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import java.util.*;

public class Bug {
    static void test(String... elts) {
        Set<String> s = new TreeSet<String>(String.CASE_INSENSITIVE_ORDER);
        s.addAll(Arrays.asList("a", "b"));
        s.removeAll(Arrays.asList(elts));
        System.out.println(s);
    }

    public static void main(String[] args) {
        test("A");
        test("A", "C");
    }
}
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would print 2 identical lines, but in fact it prints

[b]
[a, b]

The results are dependent on the relative sizes of the two sets,
violating the Principle of Least Astonishment.