This may not count since I was a CM at the time, but when I was on my college program, I was working retail at RnRc. I was at a side register, and while there, a guy came up to me with one of the logoed baseballs from our giftshop. He asked for a pen, and when I asked why, he said that Tia Carrera (from Wayne's World and True Lies) was in the back of the gift shop and he wanted to get her autograph. He pointed in the direction of his wife, so I thought he was kidding saying that she looked like Tia Carrera.
After a minute, when no one was at my register, I was wandering the store straightening up items, and spotted Tia. As CM's, we're not allowed to treat celebrities any different than normal guests. She was casually trying on hats with her friend, so I made some joke about the hat, and wandered back to my register. She later came to my register to check out and even chatted with me for a minute!
She was there because at the time, Lilo and Stitch was getting ready to premier, and it was the last animated feature actually completed at DHS, so all of the stars were there doing photoshoots and what not. She was very down-to-earth and relaxed, and honestly was mostly going unnoticed by people (I think because she didn't have an entourage or anything and was just walking around casually with her non-famous friend.
I came across a few other celebs during my time as a CM, but that one was definitely the most memorable.