It's a misunderstanding that the infallibility is enshrined in law. I'll quote a post I made in an earlier thread.

1. Historically, mechanical tools are presumed to be working well. This makes things simpler. The example quoted by the Guardian is a good one[0]: if someone wants to question the accuracy of a clock, it's on the person claiming the inaccuracy to prove their point.

2. In 1984, it became clear that computers are not just simple mechanical tools, and they were explicitly excluded from this assumption, by saying that computer evidence should be considered 'hearsay' (and therefore inadmissible) unless the prosecution can prove that the evidence is correct, either by a certificate from someone who can reasonably be expected to certify the correct functioning of that particular evidence, or by oral evidence.

3. This meant that anyone depending on the reliability of evidence from a computer (or piece of software, hardware, etc.) as part of their legal argument could be called upon to prove this, and the burden of proof lay with them (i.e.: as a defendant, I could require the prosecution to prove that the computer works as it is intended).

4. Following a review, it seems to be basically the conclusion that the requirements are inconsistent, unnecessarily onerous and time-consuming, and the way it was written was allowing criminals to get off on technicalities because the prosecution were not able to prove minor or irrelevant points about the functioning of the computer, and anyway other countries don't have any special rules about computers. You can read for yourself the recommendation here: https://cloud-platform-e218f50a4812967ba1215eaecede923f.s3.a... (starting page 200 of the document, 215 of the PDF).

5. In 1999, the specific requirement for computer evidence to be treated as hearsay was removed.

The law does not say that computers are infallible. It is still possible to challenge the accuracy of a computer system, but the burden of proof lies with the defence. It's not going to be good enough to say 'well I don't know what happened, it must be a computer glitch', and as a result, cause the prosecution to need to produce evidence that the terminal in the Post Office was working correctly, as well as all of the back end servers that may have been responsible in some part for producing the output.

There's an extent to which I think this is reasonable. If I'm accused of fraud based on evidence recovered from a bank computer, it should not be the case that I can require the prosecution to prove that the bank's computers function correctly from first principles, and the evidence be thrown out in case the prosecution are unable to do so.

The problem with the Horizon convictions is that in many cases, the evidence produced by computers was the only evidence. Also, as the Post Office has its own prosecutors, they could chase and prosecute cases which would not normally have been tried by the CPS due to lack of evidence. It's also clear that the Post Office bullied and threatened not just the sub-postmasters, but also journalists, to keep quiet about the existence of evidence which might throw into question the correct functioning of the system.

This whole debacle is not primarily caused by the principle that you're referring to. The presumption that computers function correctly has undoubtedly saved billions of pounds, hours, and allowed a huge number of successful, correct convictions, which otherwise might have resulted in not guilty verdicts due to clever litigation, rather than actual innocence.

[0]: https://www.theguardian.com/uk-news/2024/jan/12/update-law-o...