Would someone suggest how would hundreds of investigations (and courts? Or there were no courts at all?) were not able to shed any light on the bug? How is it even possible to continue for more than 20 years?
The Post Office knew that the Horizon system was deeply flawed and that the prosecutions they were pursuing were unsound. They intentionally covered up this fact. The courts adjudicated based on evidence provided by the Post Office, but that evidence intentionally omitted things like error logs that would have undermined their case. The defendants had a right to see that evidence, but the Post Office simply denied that it existed. The Post Office were helped by the fact that the rules of evidence presume that computer records are accurate unless shown otherwise - the burden was on the defence to show that Horizon was flawed, not on the Post Office to show that it was reliable.
Lots of things had to go wrong for this miscarriage of justice to happen, but at the core of it all is the fact that the Post Office lied for years - to the defendants, to the courts, to the media, to government. Things should have been handled differently, the case has revealed a number of systematic failings that need to be urgently addressed, but there's only so much the legal system can do to defend against very sophisticated people who engage in a determined campaign of deceit. We can reduce the risk of something like this happening again, but sadly we can't eliminate that risk entirely.
It's worth noting that courts (in the common law system) don't generally play any kind of inquisitive role in cases - there's a presumption that both sides bring legal representation for an adversarial debate where the court is impartial and independent.
Given many of the sub post masters struggled to fund their legal cases (and legal aid is not likely going to stretch to funding the intensity of expert witness work required to discredit a whole system), this also seems to be one of the non technical failings not often talked about.
There's certainly questions about why courts didn't spot issues, but ultimately the common law adversarial system assumes both parties can get equal quality of representation. Unless you have very strong technical knowledge (this site is a non representative sample!) you'd struggle to really shed any light on the bug and get a court to agree with your line of defence.
Ultimately that's what did happen in the group litigation by Bates, about which the TV drama was ultimately created.
Imagine you are head of some administration. Times are getting hard, profits are decreasing so you can not embezzle money as much or as easily. You know people under you must be stealing, and some new software just happen to be able to prove it. Here is the solution to help pay your third mansion in the south of France.
