The Post Office Horizon case is the largest miscarriage of justice in the UK. While voiding convictions and providing adequate compensation is an immediate priority, the technical complexity of the case and the on-going Post Office Horizon Inquiry have paralysed decision making, and distracted from the strategic urgency of addressing the poor IT culture that led to and fed the problems. Horizon is a symptom of deeply-entrenched cultural problems with IT, including poor programming causing errors, and undermining the reliability of computer evidence for use in court. Failure to understand IT led to the misleading common law presumption that computer evidence is reliable, which undermines disclosure requirements in courts and further reduces scrutiny of computer evidence. Legal reasoning on the reliability of computers in court is flawed. Throughout the Horizon scandal, the inability to distinguish naïve and dishonest IT optimism from rigorous scientific thinking and evidence ensured that incompetence knew no limits. In short, what started (put charitably) as incompetence transformed into a scandalous “delay and deny” cover-up. IT problems have a wide impact in many areas far beyond the Post Office Horizon scandal. As AI gains wider use it will create worse problems, particularly for legal evidence. Raising, debating and taking steps to manage these generic and besetting IT problems are of fundamental importance in the digital age to achieve a safe and just society.