I got to spend a couple of days at a sci-fi convention at a table next to Richard Hatch of the 1970s Battlestar Galactica fame. He was a really cool guy and we spent all day joking around. Every so often, someone would come by and say, "hey! You're uh... uh..." and I would say, "he's Dirk Benedict." (That was his BSG co-star). Once I told a lady that she better not call him Dirk Benedict because it makes him really mad.
If I had a photo of Dirk Benedict, I would have asked him to sign it.
Sadly, Richard Hatch is no longer with us. I really enjoyed hanging out with him. He was way into tabletop gaming and even would leave the table sometimes to host a session. I'm not a huge TTGer anymore, but I wish I could have had Richard Hatch as a GM.
I've had a lot of cool experiences meeting people in the sci-fi/fantasy world by working tables at sci-fi cons due to two different reasons about ten years apart, but that era of my life is over. I can only go as a regular conventioneer now. I take my daughter to them on occasion if one is nearby because she likes to look at all the cosplay.
I also got to see Gwar and The Misfits do a free double-act concert late at night at DragonCon in the 90s, which topped most of the notable people I met, even though my experience with pretty much all of them was good.