What was the UK Post Office (Horizon) scandal?

Faults in Horizon, the accounting system Fujitsu built for Post Office branches from 1999, created false cash shortfalls; on that basis hundreds of subpostmasters were convicted of fraud and theft, and thousands had to repay money that had never gone missing.

• Horizon: a counter sales and accounting system with about 40,000 terminals, which Fujitsu called the "largest non-military IT system in Europe". For its first ten years branch data was stored locally and uploaded via ISDN, and this part did not work reliably.
• Subpostmasters (Poststellenleiter), who ran branches under contract, were held liable for every shortfall the system showed.
• Consequences: false imprisonment, lost savings, bankruptcies; some died before being cleared. Not all have been exonerated (100 convictions overturned by February 2024), compensation was partial and often below legal costs, and Fujitsu kept winning government contracts.

The core failure was treating the computer's output as more trustworthy than the people operating it.