My T has told me before when he is having countertransference, although he doesn't use that word. It is helpful to me that he "owns" his reactions and says so. It makes what is happening more comprehensible to me and I think it also helps him get his own responses under control. The countertransference doesn't even have to be "mishandling," but just "is." It exists in the room, let's acknowledge it so we're all aware of what is happening here. I know my T wanted to at times give me the advice to drop kick my H across the room and bolt from the marriage and run like hell, based on his own experience of his own (past) marriage (we have some similarities). It was to his credit that he controlled that and just let me and my marriage be and find our own path and didn't give advice based on his personal experience. He would always acknowledge to me when he was struggling to hold back, so I knew what was going on.

As almedafan wrote, a big ego can get in the way of the T being able to acknowledge countertransference.