Let's see...
When I was in Disneyland in 2017, we were in line for Soarin', and in the row in front of us was Adam Scott from Parks and Recreation. I had never watched an episode at the time, but my friend who was with me was a fan and was having a mild freak out session behind me.
That same trip (it was the celebrity sightings trip) I was in line for Snow White's Scary Adventures, and Scott Patterson from Gilmore Girls was there with his family in line behind me. I kept passing him as the line was winding around, and I could tell he knew I recognized him. But I didn't bother him. Guy deserved to be able to go to DL with his family without being bothered.
When I was in middle school, I was in WDW, and High School Musical was very big at the time. I was in line for Space Mountain, and a guy got on it, and I thought, "That dude has hair like Corbin Bleu". I got off the ride and sure enough...he was surrounded by a huge crowd of people. I was too nervous to go up to him, though.
And lastly, at DHS (because yes, all of these take place at Disney Parks) when I was very young, we saw the Baha Men, the group who asked, "Who Let the Dogs Out" doing a photo shoot in front of Rock 'n Roller Coaster.