During the Atlanta Olympics, I had a crappy part-time job selling hats and T-shirts at MARTA stations. I got sent out to sell crap at the station next to the gold medal baseball game, and when it let out, the place flooded with people, and there were so many no one could move down the escalators to get to the station. And my manager looked at me, grinned, and said, "Sing."
And I went, "...What?"
And he went, "Sing!"
So I turned around, took a deep breath, and started singing "Batti batti" from Mozart's Don Giovanni (I was taking voice lessons at the time).
The entire crowd of people shut up and everyone turned to stare at me singing. It was the most amazing moment of my life, to have that many people going silent just to hear me. When I stopped, they all started clapping. This drunk guy stumbled up and said told me I was a great singer, but the one that stuck with me was an older woman making her way over, looking me dead in the eye, and saying, "Don't waste that" before she melted back into the crowd.
I did the same aria a year or so later at a masterclass my voice teacher had me do, and when I finished, the woman teaching the masterclass looked at me and said, "You ARE Zerlina" (Zerlina is the character who sings "Batti batti.")
I thought seriously about trying to go pro, but I get really bad sore throats in Spring, which would pretty much cripple me, alas. But I'm in an acapella group, so I still get to sing.
And hee hee, in a similar vein, after a rehearsal we were all walking to a train station and singing. I've got the solo in a song we're doing, "Seasons of Love," and it goes up to a high C. I busted out with the high C, and this guy was coming out of a convenience store when I did, and his whole face just lit up, so much that all of us in the group just died laughing and were like, "Oh my god, did you see his face? You just made that man's life!"
The only other thing I can think of is in high school when I read something I had written aloud--we had to write "Gilgamesh goes to the underworld after Enkidu" in the style of the poem--and then the teacher called on someone else, and he yelped, "I have to go after that?! That's not fair!" I never got the assignment back, since the teacher kept it as an example.