For me it was Jeff Kinney. I was meeting a different author I love, but there wasn't enough room downstairs and we had to go up. Turns out that's exactly where Jeff Kinney works! He's the author who got me into reading so I thought that was so cool
I was working on the Jungle Cruise at Walt Disney World, and had a VIP group get on my boat with one instruction: no Brazilian guests could share the boat. Nice enough dude, built, good lookin’ dude.
As we moved down the dock to load the rest of the guests, a Brazilian tour group saw who was there and COMPLETELY LOST THEIR MINDS. Americans, including me, had no idea who it was.
Went through the ride, they got off the boat, left via the back way.
This was similar to an experience I had years ago.
Was at a summer class at a school in NYC, and one of my classmates was a lovable idiot with a British accent who was very down to earth and nice despite often saying incredibly dumb things.
One day we're out in one of the parks for a project, and a group of middle school girls wearing uniforms lose their fucking minds.
Apparently they were foreign students from the UK, and it turns out my classmate was one of the members of a huge boy band extremely popular in the UK, but not very popular or well known here.
Weird how relative fame is.
(For example, I have several answers to the OP question ranging from dinner, birthdays, or going to private clubs with different very famous musical artists, but my personal fan experience was meeting Todd Howard at E3.)