The technical breakdown that led to the failure of the Post Office’s Horizon system.

From numerous technical issues to unskilled employees, the Post Office’s point-of-sale system was inadequate on multiple levels.

The Horizon IT scandal, often referred to as the biggest miscarriage of justice in British history, has once again made headlines due to a prime-time ITV dramatization. The resulting surge of interest has led to Paula Vennells, who served as the Post Office’s CEO from 2012 to 2019, agreeing to return her CBE. However, there is also a technological aspect to the story, highlighting the consequences of unchecked power.

Put simply, Horizon was a till. More specifically, it was an electronic point-of-sale system, or Eposs, that replaced the previous paper-based tills used in Post Offices across Britain with a new networked system. It also served as the extensive backend for these tills, connecting the entire Post Office system.

The transition from paper receipts to an electronic database was supposed to save subpostmasters time and effort, enabling them to manage their accounts with ease. For example, the Horizon system could gather all transactions over a month and calculate the expected cash balance in the Post Office’s coffers.

However, the system proved incapable of handling the demands placed upon it. The Post Office was well aware of this, as early as 1999 when trial runs exposed “severe difficulties” faced by subpostmasters, according to the inquiry into the scandal.

David McDonnell, a member of the development team responsible for the Eposs side of the project, testified during the inquiry that out of the eight team members, only two were highly competent, two were average but manageable, and the remaining three or four were unable to produce professional code.

As early as 2001, McDonnell’s team encountered “hundreds” of bugs. Although a comprehensive list was never compiled, subsequent revelations during the exoneration of subpostmasters shed light on the types of issues encountered. For instance, the “Dalmellington Bug,” named after the Scottish village where a subpostmaster first encountered it, caused the screen to freeze when confirming cash receipt. Each time the user pressed “enter” on the frozen screen, the record would be silently updated. In Dalmellington, this bug resulted in a £24,000 discrepancy, for which the Post Office wrongfully held the subpostmaster accountable.

Another bug, known as the “Callendar Square bug” after the first affected branch, led to duplicate transactions due to a database error. Interestingly, despite being obvious duplicates, the subpostmaster was also blamed for these errors.

No. Even if the system had functioned as intended, it still failed to meet acceptable standards. In a 2015 statement to the House of Commons inquiry, the Post Office claimed: “There is no functionality in Horizon for either a branch, Post Office or Fujitsu to edit, manipulate or remove transaction data once it has been recorded in a branch’s accounts.” This statement proved to be false, as the Post Office admitted four years later in a high court case.

In reality, Fujitsu staff, who designed and operated the Horizon system, had the ability to remotely access branch accounts, with “unrestricted and unaudited” access, according to the inquiry.

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