Making this its own reply because it's completely separate from my reply about my own T, but I still want to say it I guess.
It's pretty long, sorry... I've never really talked to anyone about this, and it's hard to explain without some context.

I experienced this once with a T who isn't my T. It's kind of complicated, but I know him through my involvement in a non profit related to mental health stuff, first as a participant and later as a volunteer. I've been attending his weekly meeting for over a year now, and he's seen me at my worst and at some high points, and we've had a few one-on-one conversations of a more personal nature (it turns out he cares about me... weird, right?).
When my personal therapy was starting to delve into all the "abuse" and "trauma" from my childhood that I hadn't thought about or remembered until then I was having a really hard time with it. This T was concerned after a few weeks in a row of it being obvious that something was very wrong, so I had a convo with him letting him know what was going on and that it was working through stuff, not spiraling or backsliding. I'd been vague about it in meetings, so I was also doing the "it's not as bad as that makes it sound" kind of thing, because I knew he'd always kind of assumed there had been significant abuse or something for me to end up like this, and there really wasn't anything that bad.

But during that original conversation when I told him what was going on he kind of half slipped into a therapist role with me. I sat down first and he sat down not too far away, like facing me at a 90 degree angle. I kind of said a bit more than I'd planned to, some pretty stereotypical "but maybe it was my own fault anyway" kind of stuff. Then there was silence. I was looking down. I waited. He waited. After what seemed like an impossibly long time, I glanced up at him. I only saw his face for a split second before I turned away like I'd been burned. He had been looking at me with some really intense expression that I don't understand. I still don't know what I saw on his face. It wasn't bad, it was just unbearably intense and I still don't know what it means. I'm pretty sure he knew what he was doing and that it was intentional. But I don't know what "it" is.

I hadn't thought about it in a while before this post. I still feel a weird tiny adrenaline rush when I remember it.

I like my T. This T is very good at what he does and would also be a terrible fit for me as a T because of transference stuff. I don't wish this T was my therapist instead or anything. But I've never had a moment like that with my T.
It's complicated.