I attended the first startup school (in Boston). There was a large gathering at YC's office the night before the talks. A few of us were still there when pg was ready to leave. Despite having never met any of us before, he let us stay, asking only that we double-check that the door locked on our way out.

I'd like to think that I would have the same trust in my attendees if I were him. Not a big deal, but a special moment that I'll never forget :)