This is how it works :

Gpu gets a job to work on and returns results. Those results are then verified on the CPU, and if the CPU gets the same result as GPU did, that is a valid share and is sent to the pool.
If the GPU result isn't the same the result CPU got, that is a compute error, and that share won't be sent to the pool, because its not valid.

Compute errors can happen for a few reasons: bad timings, too much overclock/underclock, too high intensity.