I'm glad I've never witnessed that - fighting in a convention, I mean @_@ I hope I never have to, but...eh.
I agree with everyone about respect and cosplay. Since most everyone does it for fun and as a hobby, people shouldn't expect that everyone who cosplays does it perfectly. People have different ranges of skills and should be recognized as such.
Anyhow, one thing that I don't particularly care for (and mind you this is just personal preference, I'm not trying to say that it's bottom-line bad) is when people combine two characters into one costume for a completely different character. An example was that I saw this girl and she looked like she was Yuna from FFX-2, but was missing a few key pieces to her costume. I didn't realize until later when I saw her posting pictures on another message board from that she was actually a combination of Yuna and Rikku. When I understood what she was going for the costume was pretty well put together, but my intial impression was that she had missed a few things. Her costume seemed incomplete to me until I realized what she'd been trying to do.
I also don't like it when people cosplay just to get attention.....