There have been a few for me:

Talking about my childhood. I had to fight hard to hold back tears.

Telling my T that I loved her. I was shaking and breathing weird.

Telling my T about my attachment and maternal transference.

Asking for an extra hug during a session as we always hug at the end of sessions.

TRIGGER*******
Telling my T about my very detailed plan on how I was going to end my life and even had a date picked out. I was scared she was going to send me to the hospital as my plan was never as detailed as that. It scared me.