The present application relates to an apparatus for verifying fragment processing related data and a method of operating thereof. The fragment shader unit is coupled to the at least one data buffer. A fragment shader unit of a graphics processing pipeline receives fragment data and records fragment processing related data in the at least one data buffer on processing one or more fragments in accordance with the received fragment data. A comparator unit coupled to the at least one data buffer compares the recorded fragment processing related data in the at least one data buffer to reference data and issues a fault indication signal in case the recorded fragment processing related data and the reference data mismatch.