is it possible to get more cupping due to excessive sanding .at what point you know that you cant sand the floor anymore

Cupping isn't caused by sanding..it is caused by moisture imbalances in the wood,usually between the subfloor and the floor itself.

Solid materials can take alot of sanding. You sanded too much when you see nail popping at the seams and can see the T&G.

Right, cupping is not caused by sanding (and sometimes sanding can even out minor cupping). Cupping is generally caused by moisture or water. Sometimes this also happens when there is not enough expansion room under the base boards and then when it gets humid there is not enough room, but this usually looks like buckling and not cupping. (Sometimes people mix up terms)

Debbie Gartner, aka The Flooring Girl
http://TheFlooringGirl.com
Westchester County NY

Ive seen parket that was re-sanded cup when they put waterborne finish on it. the floor was so thin after sanding and had so many cracks it allowed the finish to flood the floor and it cupped overnight. Good news is that after it was left for a few days it layed back down on its own.

Forgot where I learned it but if one takes a 2" strip of paper and runs a damp sponge over it it illustrates how moisture relates to wood. Try it!