Every Error object gets a new PropertyMap after a recent change to capture stack trace @ constructor call.

    var e1 = new Error()
    var e2 = new Error()

    print(Debug.map(e1) == Debug.map(e2))

prints false. Previously, the above evaluated to true. "nashornException" property is added to the instances at constructor. Note that previously error objects got exception property only when thrown -- at which these objects still got different PropertyMaps.

try {
   throw e1
} catch (e) {
    print(Debug.toIdentString(Debug.map(e)));
}

try {
   throw e2
} catch (e) {
    print(Debug.toIdentString(Debug.map(e)));
}

I think it is better to add nashornException as a @Property to NativeXYZError classes.