I downloaded the marble demo you made and played around with it. It's fun to move the marble around.

I was able to get the marble to fall through the floor by changing the direction of the camera and then running the sphere into a wall with the camera angled ~80 degrees from looking straight at the wall.

I'll try to repeate it to make sure it's reproducable.

EDIT: Yep. It fell through the floor again. Maybe a problem with the physics system?