How could the subpostmaster scandal, in which hundreds of small business owners had their lives ruined after being wrongly accused of taking money from the Post Office, have gone on for so long?
The subpostmasters were sucked into a nightmare when the Post Office installed a new accounting system called Horizon to replace old manual accounting practices. They found that their tills just weren't balancing. Some tried to top up the difference from their own money, but the discrepancies mounted until some stretched into tens of thousands of pounds.
When they asked for help from the Post Office, they were told they were the only people having trouble. They weren't alone, and neither were they in any way responsible for the huge shortfalls in their tills. But the Post Office wouldn't admit that it was Horizon, not people running small shops with Post Office counters in them, that was losing the money. Instead, it pursued more than 700 of them in private prosecutions. A number of people, including a pregnant mother, went to jail.
Yesterday, the Court of Appeal overturned the convictions of 39 subpostmasters, making this the largest miscarriage of justice in the country. There will likely be more on the way: the Criminal Cases Review Commission is reviewing another 22 cases. There are some cases where people pleaded guilty to try to make the horror they were enduring end. Even those who have had their convictions overturned today will never get back the mental trauma of losing their jobs, their homes and their reputations. Three have died since being convicted.
Boris Johnson said 'lessons should and will be learnt to ensure this never happens again', while former business minister Margot James called for a public inquiry into what happened. It is extraordinary that an organisation like the Post Office refused for so long and at such great cost to acknowledge that there were problems with its software. One of the barristers representing some of the subpostmasters today accused the Post Office of having 'positively promoted a culture of cover-up and subterfuge in the pursuit of reputation and profit'.
There are clear implications for the Post Office, but there are also important lessons for politicians to learn too. The first is that it seems the subpostmasters were persecuted for so long because it was easy to ignore them, to isolate them and to make them feel as though they were the ones at fault.
We have seen this in many other scandals that have touched politics. Grenfell is one such example. The tenants in that tower block tried their very best to get people to listen to their concerns about the safety of the building, but their complaints never got as far as they should have done. Sometimes they were ignored, and other times they were listened to but never treated with the urgency and seriousness they deserved.
It is very easy for this to happen in politics. Often those who do not have experience of campaigning, of speaking the language of Westminster and of being able to get ministers' time end up falling through the cracks for a very long time indeed.
That's one of the reasons the Post Office was able to ignore Horizon's problems for so long: it was easy to dismiss the often very humble people who were just trying to run a small business. And presumably it was easier to be convinced by the assurances of those responsible for Horizon, who knew how to talk the talk, that everything was fine.
One of the reasons the David Cameron/Greensill row sticks in the craw is not that the former prime minister was earning lots of money for a company or indeed that he was on quite such a spree of messaging seemingly everyone he had ever met in government about the now-collapsed firm. It's that there remains an imbalance of power in politics, business and wider British society. Inevitably this means that if you don't have the contacts, you can easily get totally overlooked, even when you are far more deserving of help and justice than those with the right names on their lists of advisers.
It's a lesson politicians will always need to keep learning, as it is so easy for these concentrations of power and influence to build up. But the subpostmaster scandal shows what damage can be done when power is so unbalanced that an organisation can pursue totally innocent people for over a decade.