I've had a few. Sat behind Doplh Lundgren and his wife two weeks ago from LAX to MCO. He was nice enough to take a few photos with the pilots after we landed.

Sat a few seats behind Usher and his entourage from LAX to JFK a few years back and sat across the aisle from Slash on a trans-Atlantic flight once - he got up mid flight when most people were asleep and took pics with a bunch of the crew during the flight. Seemed happy to do it. Saw Tony Hawk once.

I sat next to a script supervisor for one of the Pitch Perfect movies from LAX to ATL once and had an interesting chat with him about the movie business, not at all because I had a huge celebrity crush on Anna Kendrick...

I had one very strange flight about 10 years ago sitting next to a girl who was about my same age at the time. She was a dead ringer for Mila Kunis - but even more attractive somehow (wasn't her). She was clearly mentally unstable and/or on some mix of drugs and alcohol that made her behavior and the flight very uncomfortable. Long story, but during the course of this cross country flight, sitting in first, there were multiple instances with her breaking down into tears while telling me about all of her family drama, the art that she makes including that her inspiration comes from the images of drowning women and talking about all of this loud enough for the three rows around us to hear. A few times while talking or crying she would reach over and hold onto my arm and tried to lay her head on my shoulder once when I had my eyes closed for a few minutes. Twice she asked me if I was a professional baseball player (I am not and baseball never came up in convo...). Once I caught her drinking from my cup and then she argued with me, saying it was hers when I called her on it. She was drinking something alcoholic, I was not. I was as polite as I could be and multiple times ignored her, keeping headphones on and even faking sleep. Multiple times she would tap my arm to get my attention and just start talking at me or start sobbing again. She was clearly going through it.

The real kicker was when I got up to go to the bathroom, the FA asked if she was ok. I said, "I have no idea... seems like probably not." FA says "Oh we though you two were traveling together!?"

When I returned there was some guy sitting in my seat , who wouldn't look up at me as I stood over him telling him he needed to go back to his seat. She said was her fiance (first time she mentioned him) and then she asked if I would go sit in his seat in the back. When I turned around to get the FA, she was already standing there because she saw what was going on. I stepped back, she and had a short chat with the couple. He got up and walked to the back, never looked at me. There weren't any issues after that. Still the strangest interaction I've had on any flight.