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AI failures · Other

UK Post Office Horizon Scandal — Faulty Accounting Software Data Used to Prosecute 900+ Sub-postmasters

Verified Checked against its sources on 2 sources quoted

Where it happened
United Kingdom
Organization
UK Post Office / Fujitsu
Date
Prosecutions 1999–2015; convictions quashed by statute in 2024
Function
Government / Retail
Where it happened
Branch accounting; shortfall recovery; prosecution
System
Horizon accounting and stocktaking system (Fujitsu); not an AI system
Technology
Conventional software (not AI)
Type of failure
Other
Caught by
The person affected, A court

Summary

Horizon, a branch accounting and stocktaking system built by Fujitsu, falsely reported cash shortfalls in Post Office branch accounts. Sub-postmasters' complaints about bugs were dismissed, and more than 900 were wrongly prosecuted on the strength of Horizon's figures: about 700 by the Post Office itself between 1999 and 2015 and 283 by other bodies including the Crown Prosecution Service. Many went to prison or were financially ruined. The Post Office (Horizon System) Offences Act 2024 quashed the affected convictions in England and Wales. Horizon is conventional software, not an AI system; the case is held as an automated-system precedent.

Consequence

  • More than 900 sub-postmasters wrongly prosecuted; many imprisoned for false accounting and theft or financially ruined.
  • Affected convictions quashed by the Post Office (Horizon System) Offences Act 2024.

Who caught it The person affected, A courtFound outside the organization

Analysis

What could have been done

Checking AI output against its sources

Every factual element of AI output (citations, figures, quotations) is checked against its source before reliance.

Horizon, conventional accounting and stocktaking software rather than AI, falsely reported cash shortfalls, and more than 900 sub-postmasters were prosecuted on the strength of its figures.

A way for affected people to challenge the decision

People affected by an automated decision can challenge it and get a human re-review.

Sub-postmasters complained about bugs soon after Horizon falsely reported shortfalls, and their complaints were dismissed; the affected convictions were later quashed by statute.

What would likely have prevented it

  • Before treating a system-reported shortfall as evidence against a person, establish from independent records that the shortfall is real and not a system error.
  • Give people whose accounts are charged on a system's figures a route to challenge them that includes independent examination of the system's data.

What would likely have caught it earlier

  • Log complaints about system-reported shortfalls across all branches and investigate a recurring pattern as a possible system defect rather than as separate individual cases.

Related reading Treat AI Output Like an Unsupported Journal EntryMost Documented AI Failures Were Found by Someone ElseWhen the System Is Presumed Right

Key dates

  1. October 16, 2026Checked against sources
  2. October 18, 2026Checked against sources

Developments are added when the record is reviewed. Next review: October 16, 2027.

Evidence

Each passage is quoted from its source. The quoted passages are re-checked against the live pages every month.

  1. More than 900 sub-postmasters were wrongly prosecuted because of incorrect information from the Horizon computer system. Many sub-postmasters went to prison for false accounting and theft, and others were financially ruined.

    Supports: Number prosecuted; imprisonment and financial ruin

    Horizon was developed by the Japanese company Fujitsu , for tasks such as accounting and stocktaking. It was introduced by the Post Office in 1999. Sub-postmasters quickly complained about bugs in the system after it falsely reported shortfalls

    Supports: What Horizon is and who built it; false shortfalls; bug complaints

    The Post Office itself took many cases to court, prosecuting 700 people between 1999 and 2015. Another 283 cases were brought by other bodies, including the Crown Prosecution Service (CPS).

    Supports: Who prosecuted and when

    Post Office Horizon scandal: Why hundreds of people were wrongly prosecuted BBC News Reporting or analysis

  2. Every conviction to which this Act applies is quashed on the coming into force of this Act.

    Supports: Convictions quashed by statute

    Post Office (Horizon System) Offences Act 2024 legislation.gov.uk Primary source

Suggested citation. Eminence Scan, “UK Post Office Horizon Scandal — Faulty Accounting Software Data Used to Prosecute 900+ Sub-postmasters”, Eminence Growth Solutions, last verified October 16, 2026, https://eminencegrowthsolutions.com/tools/eminence-scan/record/fail-048/.

Using our research: you may read and quote Eminence Scan with credit to “Eminence Scan, Eminence Growth Solutions” and a link; all other rights reserved. Terms: Using our research.

Spotted an error, or a development we have missed? Report a correction. How we check records: research methodology.