I found a bug when I was playing Remedy's stage on challenge mode. On the first of the duets (the A one specifically.) If you make a mistake in the middle of it, next time on the second go around, the lights still show up as the smaller error lights even though you played the right notes. You can still progress, despite the visual indicating that you messed up.

Even though it's been a while since I last beta tested, it's still a fun game. I still got that rush even though know I got through it before.