per Joshua Bloch:

public class Size {
    private enum Sex { MALE, FEMALE }

    public static void main(String[] args) {
        printSize(new HashMap<Sex, Sex>());
        printSize(new EnumMap<Sex, Sex>(Sex.class));
    }

    private static void printSize(Map<Sex, Sex> map) {
        map.put(Sex.MALE, Sex.FEMALE);
        map.put(Sex.FEMALE, Sex.MALE);
        map.put(Sex.MALE, Sex.MALE);
        map.put(Sex.FEMALE, Sex.FEMALE);
        Set<Map.Entry<Sex, Sex>> set =
            new HashSet<Map.Entry<Sex, Sex>>(map.entrySet());
        System.out.println(set.size());
    }
}

Predict the behavior of this program. Then run it and see what happens. Only two Map implementations display this disturbing behavior: EnumMap and IdentityHashMap. I'm less disturbed by IdentityHashMap, as it's such an esoteric implementation. But I could easily imagine someone getting hurt by the behavior demonstrated by the example above. Fixing it would decrease the performance of iterating over entrySets of EnumMaps, but I think it would probably be worthwhile. I violated the principle of least astonishment here, and engaged in premature optimization. Mea culpa, twice.