Doorknob therapy has been a known phenomenon since almost the start of the profession - Jacques Lacan noted it and to combat it he would not have regular-length sessions and would not tell the client how long they would be: they would range from a few minutes to an hour, so that the client would just spit it out since they didn't know how much time they had.

I did this once accidentally with No. 1 - it wasn't an event but a thought I'd been having. She didn't seem annoyed with me but annoyed that she couldn't help me deal with it in the short time we had left. It's entirely possible that was the reason your therapist seemed frustrated.