English law assumes a computer was working properly. That assumption prosecuted nine hundred sub-postmasters, and it is still the law.

Britain once took the other view. Section 69 of the Police and Criminal Evidence Act 1984 said that if you wanted to rely on what a computer produced, you had to show the computer had been working properly. The burden sat with the party using the machine.

Parliament repealed that section in 1999, and the change took effect in 2000. It followed a Law Commission recommendation, and the reasoning was practical rather than careless: computers had become ordinary, courts were spending time on technical arguments that almost never went anywhere, and the common law would presume a machine had operated correctly unless there was evidence that it had not.

That single change moved the burden. From the person relying on the computer, to the person saying the computer was wrong.

Then Horizon. Between 1999 and 2018, more than nine hundred sub-postmasters were prosecuted for theft, fraud and false accounting over shortfalls in their branch accounts. In Bates and others v Post Office Ltd, judgment given in 2019, Mr Justice Fraser found that the Horizon system contained bugs, errors and defects capable of producing exactly those shortfalls. Parliament eventually passed an Act in 2024 to quash the convictions, because the ordinary appeal machinery could not cope with the number.

The popular telling is that the Post Office lied, and it did. But the law was the other half. A sub-postmaster who wanted to argue the system was wrong had to prove it, about a national computer system they could not inspect, on an income that did not stretch to expert evidence. The presumption did not create the scandal. It made the scandal survivable for the people running it, and unsurvivable for everyone else.

On 21 January 2025 the Ministry of Justice opened a call for evidence, running twelve weeks, on how courts should treat evidence generated by software. The minister of the day said that accepting digital evidence with no questions asked can wreck lives. As of last month the department's position was that it is still considering the responses. The presumption remains the law of England and Wales.

Now the limits of the parallel, because they matter. Horizon was one system, run by one organisation, producing account balances. What the Home Office proposes is different in kind. An AI summary of four million documents is not a total that can be recalculated; it is a judgement about relevance, it may not produce the same answer twice, and its characteristic failure is not a wrong figure but a missing one.

Where the rhyme is exact is the burden. If a defendant says the summary left out the thing that would have helped them, somebody has to establish that, against a system they did not build and cannot see. That was the question in 1999, it was the question the sub-postmasters could not answer, and it is the question the review opened in 2025 was asked to settle. It has not settled it yet, and the machines arrive in 2027.

Why it matters here: none of this says the same thing will happen again. It says that the last time Britain made a decision like this one, it looked sensible at the time, and it was twenty years before anyone could prove otherwise. The people who paid for it in between were the ones with the least chance of being believed.