It happened to us in 2011 on our first trip. Totally unexpected, and I sent a huge complimentary email to the guest feedback inbox, and actually got a call from Disney about it, letting me know the CM in question would be recognized as a result of my email.
Here's our story:
We decided to take our kids to Disney for my daughter's 5th birthday. It was a surprise trip...the kids had no idea they were going until we woke them up that morning. Prior to the trip, we'd introduced our daughter to Star Wars, and she LOVED it from day 1. When she found out that Darth Vader is at Disney, she decided that she was going to write him a letter to give to him one day when she met him. She kept this letter for months, not knowing that we had our first trip to Disney coming up. In her mind, it was for some unspecified time in the future when she'd meet Vader at Disney. The morning that we left on the trip, her #1 concern was that she had her letter!
Anyway, so we go to DHS and we sign up for JTA. When she shows up for her show time, I tell the CM checking kids in (his name was Dustin, will never forget) that my daughter had a letter for Vader, and she handed it to him and asked him if he could give it to Vader. I expected him to take it and tell us that he gave it to Vader. Instead, he just told us to keep it and said that he'd see what he could do.
Next thing I know, he's pulling me aside telling me to hang out after the show, but not be obvious and not tell anyone else that we were asked to stay. After the show, he took us into the handicapped entrance of Star Tours, where we waited in the area overlooking the main queue. A Photopass photographer came out, and then a couple of minutes later, in comes Vader looking very...Vader-ish. My daughter had her back to him, so we told her to turn around, and when she saw him, her eyes got HUGE and she turned and RAN in the opposite direction, then turned and threw her arm out at him to "use the Force." Now, in real life, nothing happened. Everyone just laughed. But when SHE tells the story, she says that Vader flew up against the wall and was defeated. LOL. She sidled up to him, handed him the letter, and we took a few pictures. They also gave us an autographed light saber, which was pretty cool.
So, that's our story. Now that's some Disney magic right there. And they totally sucked us in...a week from now, we'll be on our 6th trip in 4 years.