If you are using Apple Qmaster with cluster storage and an error occurs, some partial files may be left on the designated cluster storage location. Check the cluster storage location to make sure that no partial media files are left there. If you find any, delete them and submit the job again.
This is more likely to occur if you're using Compressor and Apple Qmaster together.
(You can install Apple Qmaster from Final Cut Studio, DVD Studio Pro 4, or Shake 4.)