CGMiner litecoin mining generates invalid nonce errors
In my experience scrypt mining on GPUs is quite susceptible to cause HW errors and wrong nonces if the parameters are not chosen to match the GPUs the algorithm is run on. As the SCRYPT-README file of cgminer states:


  The reason this is crucial is that too high an intensity can actually
  be disastrous with scrypt because it CAN run out of ram. High
  intensities start writing over the same ram and it is highly dependent
  on the GPU, but they can start actually DECREASING your hashrate, or
  even worse, start producing garbage with HW errors skyrocketing. Note
  that if you do NOT specify an intensity, cgminer uses dynamic mode
  which is designed to minimise the harm to a running desktop and
  performance WILL be poor. The lower limit to intensity with scrypt is
  usually 8 and cgminer will prevent it going too low.


The --benchmark option you specified in your command line tells cgminer to attempt to find an optimal set of parameters, so it will make a few attempts with high intensities as well, which will fail. Once you figured out a good value for the parameters you should be fine.