Meeting the guys at Palladium Games, buying Rifts and getting the core book by Kevin Siembeida and Kevin Llong signed is certainly one of them. They were very approachable guys (even if Kevin S has turned into the C&D Nazi in later years). GenCon lead to the infestation of Rifts in my group for a long time, even if I was never the one to run it.
It's also where I bought Dangerous Journeys, EGG's first major RPG after getting the boot from TSR (Cyborg Commando is far from a "major" RPG).
Then there was the SpellJammer set up that was being run by Slade Henson when I found it. It was a large, space asteroid thing with dozens of miniatures and terrain elements. You chose a miniature and wandered around until you got killed - or something like that. It was impressive.
I observed for a few minutes before jumping in as another player died. I wasn't sure exactly what was going on, but Slade was really hamming it up, so whatever it was, it was fun.
Suddenly, Slade turned to me and said (I'm paraphrasing after 20 years): "You think you can handle running this for 5 minutes? I gotta take a leak!".
That shit-ton of panic hit. How the fuck am I going to run this? I don't even know what the rules are.
"It's simple - high roll on a D20 wins - one hit kills. Play it up, be descriptive. I really gotta hit the can. Just 5 minutes."
So I did it. I actually have no memories of how well I did. I think I was too damn nervous to be able to remember. I do recall Slade was gone for over 20 minutes. It apparently was a bit more than a piss break ;)
In any case, he thanked me and I moved on to the next shiny in the room. But I can say, for about 20 minutes, I ran a TSR event at a GenCon. I just wish I could remember the details ;)