First if anyone could formulate how to be in relationship, then we would be all millionaires.
That being said one thing I notice as someone with traumatic background but highly successful and functional not to horn my horns but also educated about human relationships to a point is thus:
Just as the comment above about being annoying or feeling of annoying and answering that as two adults in the room... Not as regression assumption that the client might be acting little boy or girl but is trying to integrate child parts with adult parts in the session.
So validation of uncomfortable feelings in extreme cases even emphasizing with rage and hostility so the person can feel... Still I am accepted and not take those feelings outside of therapy.

In the above comment, if I was therapist, I would nt just say no cause that is rude even in a normal setting but...would explore how my change of mood induces this person feel annoying... So I may said I am sorry you feel annoying when I am little off and I am glad you tell me. I do not find you annoying but we could learn something together about what reminds you (depending the strength of alliance) annoyance when I am off...
And another time as education and relating to the adult part would say... It could be coincidence when I am tired, hard day etc you feel that...
Etc etc
The problem the therapist minimizes her feeling or the impact of appearing cold and over emphasis the annoying feeling of the client being theirs alone.
Now the client is here looking for answers.
Of course therapist is human and having a bad day is allowed but clients are buying emotional space so bring it up next time you feel better....
Also this good in any mature relationship...
Adukt to child is over emphasiized versus sprinkling with adult to adult aka alliance relationship.

Missed opportunity.