Why was faulty software that caused the Post Office scandal not tested?

AS a retired accountant who spent much of much of his career in systems development and implementation, I read Andrew Tickell’s article in last weekend’s Sunday National (Testimonies of ripped-off Post Office staff highlight reckless greed, May 15) with interest and mounting rage. Not, I hasten to add, at his excellent reporting, but at the egregious criminal behaviour of the Post Office and the developer of its now notorious software, Horizon. Source