What wrong with this Groovy method?

def setSymmetricDifferenceTheWrongWay(s1, s2) {
    def sd = s1;
    sd.addAll(s2);
    def tmp = s1;
    tmp.retainAll(s2);
    sd.removeAll(tmp);
    return sd;
}

Remember Lists are still just objects in Groovy as the are in Java, not primitives. Assigning a new reference to an object does not copy the object. You need to clone them.

def setSymmetricDifferenceTheRightWay(s1,s2){
    def sd= s1.clone();//remember these are objects, assignment without clone is not copying
    sd.addAll(s2);
    def tmp=s1.clone();
    tmp.retainAll(s2);
    sd.removeAll(tmp);
    return sd;
}