What ever you do, DON'T TOUCH THAT LENS without a lens cleaning kit. You have a higher chance of totally ruining your GameCube by touching it... Unless that's the problem. Maybe you or someone touched/scratched the lens by mistake.
I know it's highly unlikely, but maybe you didn't 'fully' place the games in both times. I remember one time I was getting the same message with SMB, so I took the disk out, and put it back in, but got the same message. I opened the lid, and pushed the disk down slightly until it would move no more, switched the system on, and it worked fine.
__________________
|