It's called a vegetative center. It's not a big problem. Sometimes over fertilization can cause it and there are some roses that are more prone to it than others. I've actually had a rose grow another bloom from the center of the first bloom. It looks weird but it won't hurt the rose. The next blooms should be fine.

Seil nailed it. Also called "proliferation" if you are searching the web.

I've had those on one of my minis before. It was really hot that summer when it had proliferation but I didn't know the actual cause. I kinda like it though. Reminds me of viridiflora on a smaller scale. The only bad thing is I can't pollinate nor collect any pollen when it happens since the all the reproductive parts transformed.

Sometimes you see this in the spring when the bushes are growing fast. I even had a two headed bloom once. Just cut it back and it will produce a nice bloom the next time