British Post Office scandal

The most human entry here. Fujitsu's Horizon accounting system, rolled out across Post Office branches from 1999, showed shortfalls that subpostmasters could not explain, and more than 900 of them were prosecuted between 1999 and 2015 on the strength of its figures. In Bates v Post Office in 2019 the High Court found Horizon had contained bugs, errors and defects, and it emerged that Fujitsu staff could alter branch accounts remotely. It is the starkest lesson on this list about treating computer output as proof. In May 2024 a new law quashed the convictions in England, Wales and Northern Ireland. Anyone building systems whose records can be used against people should know this case.