I had it happen to me on out last trip, and I was really unhappy about it.
We were in the butterfly garden in EPCOT, and at the end of it was a really adorable group of topiaries of Bambi, Thumper and Flower. My younger son really liked the topiaries and asked me to take his picture in front of them.
This was huge, because my younger son is extremely hard to photograph. He is not a fan of having his picture taken, is a combination of self-conscious and impatient, and even when he is posing is very hard to get to actually look at the camera. I was so happy that he had asked for this shot, because it meant I would have one picture of him that day actually looking at the camera and smiling.
There were quite a few people waiting to take pics, so we waited a while, and finally it was our turn. Both my boys got into position, I raised my camera, they both looked straight at me and smiled, but before I could click the shutter some obnoxious woman flung herself into the picture, throwing her arm around my younger son and saying "cheese!"
Apparently she was wearing the same shirt as them, and decided that the most HYSTERICAL way to point it out (to her, anyway) was to freak out my kids and ruin my shot.
She hung around for a few long seconds, and then finally left without apologizing. I tried to get the boys to focus again but of course the moment was gone. The picture my little one had been so proud and eager to take was now spoiled for him, and for me. He didn't ask me to take a single other picture of him again for the rest of the trip.
I know it wasn't the end of the world, but it was a selfish, obnoxious act, and it totally took from me something I had been thrilled about moments earlier.
I never did get a good shot of him that day, and every time I look at the confused & distracted shots I did get of the boys after her obnoxious stunt, I get annoyed all over again.