Nothing has really gone wrong with the bike.
I bought and put in a new cam chain when I first purchased the bike, but I knew it needed it when I bought it.
I have had a puncture and needed to replace the rear brake pads, oh, and the speedo cable, but that's it.
The bike is in great condition. I have had two of these previously, but this one is far superior to the others in condition. I had a guy ask if it was new at the lights the other day in fact, "Not quite" I told him.
As I have said, this is my third GPZ600r and for the money I think you would be hard pressed to get a better bang for your buck.
Including the other two previous GPZ's, I would have clocked up over 50,000km's on this model bike and find it to be comfortable, economical, cheap to own and run, and a great little bike. It's not the fastest thing out there, not by a long shot, but plenty quick enough to have fun, and in the right hands will not get to left behind by newer stuff either. I am very happy with it overall.