An important problem for computer-based systems is providing fault tolorance for unknown (at the time of commencement of service) systematic design errors. Such design errors can have a long latency in normal operation and only become apparent under specific conditions associated with particular combinations of input and internal system states. The use of 'divers' software versions remains a possible approach to prevent coincidental failure, but its potential value has never been quantified. This paper presents the application of data-flow and constant perturbation to simulate the introduction of faults or errors into programs and explores methods to establish the magnitudes and locations of the associated input space failur regions. Used together, these two techniques enable failur behaviour to be described in a quantitative way and provide a method to estimate the diversity of multi-version software. A simple case and a industrial software are studied to illustrate the applications of the approach.