By default your validator software picks the unmodified chain unless you install a special update that lets it use the newly forked chain. If you do nothing you

By default your validator software picks the unmodified chain unless you install a special update that lets it use the newly forked chain. If you do nothing you continue mining just one chain (the old one) and nothing is lost. If you validate with both then you are penalized. That can't really be done on accident.