What lessons can we learn from the Post Office scandal?

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.