The WU (work units) are used to calculate the number of HW errors. Let's assume your card mines with an average of 20 MH/s and has an error rate of 2%. Then the expected WU value would be 20/m * (1-0.02) = 19.6/m. The hardware errors are normal, but they aren't shown by other Ethereum miners. A defective card typically produces 10 times more hardware errors then a working one.