Hi and welcome :) It should be a mistake ,while because of NPE( Non Parental Event) a son and a dad could be from different groups for obvious reasons, it is not possible for mom and son, please do not be worried and upset and you could just take a test in another company, also what kind of relationship do they give you between you and your Son at 23andme? :) Genetics is just science and it is not perfect, no room for worries :)