There were two that I can remember. One was said about my friend, another almost got a guy decked.
The first one, I was doing a group cosplay. We did a fun little tweek with two of the characters and j-rocked and lolita-ed them somewhat. Well, as I was walking by, I heard someone mutter "I don't understand why she's cosplaying with failures like that." I stopped, glared at the person, and stormed off. I was very upset with that whole ordeal.
The second one happened at Comicon 2007. There was a booth babe for the Quentin Terintino (forgive my spelling. I'm not much of a director person) that was an actual amputee with the gun as her leg. I was so amazed at the fact that she was willing to being in the public's eye, knowing full well that everyone was looking at her lack of leg. As I was floored at her guts, I heard a guy, rather loudly, say "They have the gun on the wrong leg."
Er..wait.. what??? I'm sorry, let's cut off her other leg, replace the old one, and put the gun on the new stump! I almost decked him for being heartless like that.