A couple of times my T has told me when he is having countertransference. Saying it outloud has a way of disarming it. That way it is out on the table and I know if my T is struggling momentarily with his own issues. For example, when I was going through my divorce, T listened to my description of something that happened recently with my husband and was silent a while and then shared that he was struggling not to say something to me that was based on his own past situation in his marriage/divorce rather than on mine. He was reminding himself that he was not me and that my marriage was not like his had been. It was fine. I appreciated his honesty. It did no harm. I think what it is harmful is when a T doesn't realize they are having countertransference and proceeds willy nilly through the session as if it is about the client when it's not.