Towards the end of the Clone Wars, Palpatine set up secret cloning facilities on Centax-2, a Coruscant moon. Arkanian Microtechnologies, the corporation that ran these facilities, used Spaarti cloning technology. The first batches of Centax troopers were being incorporated into the GAR shortly before Order 66 and the declaration of the Empire.The Spaarti cloning technology was never used for the production of clone troopers during Palpatine's rule. [...]
It was discovery of increasing numbers of these inferior clones that led a small group of Fett clones to put into action a plan to desert.