I saw him speak at a startup event in 2010 at MIT called Startup Bootcamp. It was probably my first startup-related conference and he was the first talk in the kick off slot at 9am. He gave a great talk recapping the origin of GitHub and how it grew out of another projected called FamSpam, a social network for families.
After the talk I had to run to the restroom and happened to run into Chris out in the entry way. I introduce myself and we starting chatting. As we were talking, people started walking into the event late. They saw us standing in the entrance, and started asking questions on where to go.
Instead of deferring responsibility to someone working at the event, Chris sat down at the empty welcome table and started checking people in by giving them schedules and helping them create name tags. We ended up checking in a few dozen people together while we talked more. No one knew who Chris was when they walked in, and just assumed he was a member of the event staff. I think had they known he was the co-founder of GitHub they probably would have paid more attention to him.
I ended up sending him a t-shirt and he took the time to shoot me back an email saying thanks. The subject line was "Dude" and the text was "Got the shirt. It's so awesome. You rock. Tell your brother yo, too!"
Anyways, I just thought it was kind of cool he took it upon himself to help out with checking people in at the event even though he had volunteered to travel all the way to Boston to speak for free to help out young, aspiring entrepreneurs by sharing his learnings. It always kind of stuck with me that you need to stay down to earth and pay it forward no matter how successful you get.