I understand that every node audits the computational work done by the miners separately. If we want the computation speed to scale well, we cannot have each node doing every calculation himself. Instead, without any incentive, a node will just pass along a block without verifying the computation. However, if he has some incentive to find an error in the computation, then he can use a probabilistic method of finding errors quickly. Competition for finding errors will lead to almost certainty of no errors in the computations.
Is this being done already?