The only thing is, it doesn't happen only in duels with Fire Cat. My sister was on her lower leveled balance wizard when it happened to us for the first time. I personally did not see this, but this is what she said. She was walking along Unicorn Way, minding her own business on the sidewalks, when it happened. She was going to battle Rattlebones for a second time. Suddenly, the big purple box appeared in the corner of her screen. When she turned to look at it, it dissappeared.
This happened again when she was dueling with Rattlebones. I was watching her for this one. It just magically appeared when Rattlebones cast a spell. Neither of us can remember what the spell was, but based on what everyone has said it was probably Fire Cat. By the time the animation of the spell was over, the cube was gone.
It would be really nice if someone would fix this!
Another thing about this. Whenever I see someone cast Fire Cat, I never see this on my character. I've only seen it on my sister's balance wizard. Maybe that is because my level 32 ice wizard, my main character, has never cast Fire Cat as far as I know.