A digital-based control data processing system detects during system operation the occurrence of data processing errors. Data processing accuracy is verified by receiving output information developed from system input information in accordance with a first operational function, computing a derived version of the system input information in accordance with a second operational function, and comparing the derived version of the system input information to the actual system input information to determine whether they differ by a predetermined operational tolerance. This method for verifying processing accuracy is especially advantageous for detecting latent software errors which are unique to a particular computer program. The processing system includes display apparatus which develops from the output information a set of display symbols that convey the information on a monitor screen to the pilot. Assigned to each display symbol is an identifying tag which identifies in real time a display symbol that conveys erroneous information.