All of the postmasters were legally guilty of false accounting because, when they rang the support line after seeing that there was an alleged discrepancy of £X, they were told that they had to accept the amount of the shown shortfall and that the post office would sort the problem out, and they did press the button to accept it, they had no choice because the system locked them out until they had accepted it, ergo they agreed to a false figure and were therefore guilty of false accounting.
Then, when the post office were forced to do something, due the number of cases, their response was to set up an "independent inquiry" that was in fact entirely staffed by their own employees, employees of the faulty software firm and tame union members, so not independent at all. Their conduct was disgraceful and if it hadn't been for Computer Weekly and then Private Eye, they would have got away with it.
But, really a problem with dishonesty and arrogance rather than with technology.