Josh Bloch writes:

Matt Shulman at Google discovered a bug in EnumSet: Invoking
retainAll on an empty EnumSet can return the wrong result if the
argument is an EnumSet with a different element type. Needless to
say, he discovered this by reading the code.

Here is a regression test, all ready to go (assuming you're still
using the same testing tool):

import java.util.*;

public class EnumSetRetainAll {
    enum RegularA { A0, A2 }
    enum RegularB { B0, B1 }
    enum JumboA {
        A00, A01, A02, A03, A04, A05, A06, A07, A08, A09,
        A10, A11, A12, A13, A14, A15, A16, A17, A18, A19,
        A20, A21, A22, A23, A24, A25, A26, A27, A28, A29,
        A30, A31, A32, A33, A34, A35, A36, A37, A38, A39,
        A40, A41, A42, A43, A44, A45, A46, A47, A48, A49,
        A50, A51, A52, A53, A54, A55, A56, A57, A58, A59,
        A60, A61, A62, A63, A64, A65, A66, A67, A68, A69
    }
    enum JumboB {
        B00, B01, B02, B03, B04, B05, B06, B07, B08, B09,
        B10, B11, B12, B13, B14, B15, B16, B17, B18, B19,
        B20, B21, B22, B23, B24, B25, B26, B27, B28, B29,
        B30, B31, B32, B33, B34, B35, B36, B37, B38, B39,
        B40, B41, B42, B43, B44, B45, B46, B47, B48, B49,
        B50, B51, B52, B53, B54, B55, B56, B57, B58, B59,
        B60, B61, B62, B63, B64, B65, B66, B67, B68, B69
    }

    public static void main (String [] argv) throws Exception {
        Set<RegularA> ras = EnumSet.noneOf(RegularA.class);
        Set<RegularB> rbs = EnumSet.noneOf(RegularB.class);
        if (ras.retainAll(rbs))
            throw new Exception("Regular failure");

        Set<JumboA> jas = EnumSet.noneOf(JumboA.class);
        Set<JumboB> jbs = EnumSet.noneOf(JumboB.class);
        if (jas.retainAll(jbs))
            throw new Exception("Jumbo failure");
    }
}