The other day I was telling him about a huge fight with my brother that's the reason I'm no longer welcome at my mom's house. My brother explodes at the littlest things and I was always blamed for his verbal and physical abuse because I "antagonized him."
I was saying how I was visiting for Christmas a few years ago and my twin brother had left a grill on and forgotten about it and was about to go to bed. I was frustrated that he totally blew me off when I pointed it out. He said "this was the only time I've done that" and I said "great, the house will only burn down once then."
T accidentally burst out laughing at that even though it was in the middle of a really serious topic. He got a lot more serious when I told the second half of that story...
In a way him laughing was kind of a relief though. I know I was being a smart*** but at the time everyone acted like that had warranted my brother's explosion. It meant my T didn't think what I'd said was too serious or awful.

I can't specifically remember any of the other times right now because it's been a while. Our sessions have been pretty serious lately, so there hasn't been much teasing or joking.

It helped establish some rapport early on because it showed his more human side. I don't remember exactly what I said, but the first time he laughed he was laughing so hard that he apologized for not being able to stop laughing and he tried to get serious again and then started laughing again.
It might have been the same session when I said I was worried that no one would care once I didn't need help anymore and he said "don't worry, I think you'll need help for a long time" which added some levity and made me laugh.

Edit (SH TW)