My sightings involve only minor "celebs":
1) Playing pick-up basketball at Arizona State in 1979, I watched Lafayette Lever, future NBA All-Star, and his teammate Johnny Nash race up and down the court. They were in a completely different league (no pun intended) and I didn't dare try to get into the game.
2) Same summer, a 5'5" acquaintance came over to sit with me at lunch and brought along her friend Alton Lister, a 7' future center for the Milwaukee Bucks. Both were very nice, but they made an interesting couple.
3) A decade later, playing pick-up b-ball with athletic department faculty at the U of Houston, one of their alums joined the game. I don't remember the name, but he had been a starter on their national championship team and at the time, was 12th man on the LA Clippers, the worst team in the NBA. Only once did he really dial it up. Usually he went about 1/3 speed and still did whatever he wanted. His wife made us look silly.
4) Went to high school with a starting linebacker for the 49ers during their championship years. He was a solid player, though not really a celebrity, I suppose. He was a very smart guy in high school, but someone to avoid on game day when he was getting "psyched up".
5) Heard Linus Pauling speak and later met him. I've met several other Nobel Prize-winning scientists who don't really count as celebrities.
I guess I've led a sheltered life. Are they really celebs if I need to explain who they are?