I am unclear about whether money actually went missing, or whether transactions were 'created' that never happened leading to a deficit in the accounts? Physically since the local PO is a mostly cash business, missing money would have to have been a physical local action.
It appears that the accounts could have been altered remotely, but to create a negative balance this would have required creating a record of transactions that never occurred. Has anyone identified examples of these factitious transactions? Is there any suggestion as who was remotely accessing the accounts to do this?
What I haven't seen (and I may have missed this) is any suggestion the software had a glitch that spontaneously created transactions.
Mostly, there was never any actual money missing.
https://www.theguardian.com/uk-news...s-horizon-system-failed-a-technical-breakdown
If you don't want to read the whole thing, here are a couple of salient points
What sort of bugs resulted?
As early as 2001, McDonnell’s team had found “hundreds” of bugs. A full list has never been produced, but successive vindications of post office operators have revealed the sort of problems that arose. One, named the “Dalmellington Bug”, after the village in Scotland where a post office operator first fell prey to it, would see the screen freeze as the user was attempting to confirm receipt of cash. Each time the user pressed “enter” on the frozen screen, it would silently update the record. In Dalmellington, that bug created a £24,000 discrepancy, which the Post Office tried to hold the post office operator responsible for.
Imagine you have just entered a bunch of transactions that total $550. You press the
Enter key to finalize the transaction, and nothing happens. You press it again, and again and again. Finally, the screen posts up the transaction and shows you the details. The customer hands you the $550, and you put it in the till, you print the receipt and hand it to them. But unknown to you, every time you pressed that
Enter key, the whole transaction was recorded each time - total $2,200, but you have only taken $550 from the customer. You now have a shortfall of $1,650, and it doesn't take many of those in a week to have a shortfall in the tens of thousand of $$$
But in some cases, you didn't even have to have multiple presses of
Enter
Another bug, called the Callendar Square bug – again named after the first branch found to have been affected by it – created duplicate transactions due to an error in the database underpinning the system: despite being clear duplicates, the post office operator was again held responsible for the errors. 
So not even pressing the
Enter key could still result in multiple transactions being recorded.
