I will always wait till the next session to bring it up. My therapist starts each session with this 4 question questionnaire thing about how he did on the previous session so it's easy for me to write him low on something and he knows right away that something has upset me. Sometimes I'm able to talk about it other times I have something written down and hand it to him to read. He is very good at apologizing when necessary and we usually talk it out and everything is better by the end of the next session.