I know that for the larger grading/slabbing firms, grading is by concensus, I believe that 4 different graders give their opinion on each coin. The grades then go to the finalizer who issues the grade. I guess this type of fraud is possible, but it would also require collusion of several different individuals.

There is/was another kind of fraud with the earlier slabs. Individuals would remove coins from a slab and reinsert a similar coin with a lower grade. They are now making the slabs tamper proof, so this type of fraud will be more difficult.