A processing circuit has functional units (10a-c) configured to perform operations each in response to a respective command. The functional units (10a-c) are configured to execute at least one of the operations with a selectable level of susceptibility to incurring an error during execution. Different functional units may be provided, designed to execute the same operation with different levels of susceptibility at the cost of more circuit area, power consumption or execution time in the case of less susceptibility. The less susceptible functional unit may comprise additional error correction circuits, or more pipeline stages for example. The program directs commands to execute the operation to different functional units according to the required level of susceptibility. High level programs may be provided wherein variables are declared with a specified level of reliability. These declarations may be used during compilation to select how instructions will be executed.