Which technical and development failures lay behind the Horizon shortfalls?

Bugs such as the "Dalmellington bug" that duplicated transactions, an asynchronous design in which messages could overwrite local data, remote data changes by Fujitsu that looked like the subpostmaster's own, and a development process without design documents, reviews or coding standards.

• Dalmellington bug: the screen froze while cash receipt was confirmed, and each press of "enter" recorded the transaction again (£24,000 discrepancy in Dalmellington).
• Remote access: Fujitsu could alter branch data as if at the branch, and it appeared as if the subpostmaster had done it. The Post Office had publicly denied this for years.
• Asynchronous store-and-forward: messages to and from the centre could trip over each other and overwrite local data.
• Process: "no design documents, no test documents, no peer reviews, no code reviews, no coding standards", testers raising bugs "by the thousand", and a team without relevant qualifications.