[petek]

Should the ball stay in the same position and orientation in the cup, or do you need it to move? You probably are using a tangent constraint, but that will not keep it from flipping out, so you might have to constrain some other features or planes (like the Origin planes or axes).

Doing that will limit (or prevent) motion, though. Usually not a problem for static designs, but might keep you from running constraint-driven animation.

Like Greg suggests, a picture would be helpful.