I've encountered what I think is a bug in Super Mario Galaxy 2. I found it by...dying. A lot.
And I don't think this is a level-specific thing either. It just happened to me in world 6, in a stage where you use Rock Mario (awesome, BTW) to continuously roll through an insanely difficult obstacle course floating in space. It was mostly me falling off platforms, as Rock Mario is easy to oversteer.
The "bug" may just be a counter thing. In this stage, they've given you plenty of opportunities to gather star bits to earn extra lives. It's like they knew how difficult this stage was. The case here is that I died enough times to gather so many star bits that I took the counter to 999. Yes, tell me how much I suck. It took me a couple of turns (deaths) to notice that the counter wasn't increasing over 999, costing me valuable extra lives.
As you'll see in the video, this stage is a real bitch. I did finally beat the stage, and followed up with the loudest, most obscene victory dance ever. That part was not recorded.