That goes back to 1999, when the Horizon software system was installed in post offices by Fujitsu subsidiary International Computers Limited. From 1999 to 2015, Fujitsu's faulty accounting software aided in the prosecution and conviction of more than 900 sub-postmasters and postmistresses who were accused of theft or fraud when the software wrongly made it appear that money was missing from their branches.

During the prosecutions, courts hearing cases against postal employees "were not told of 29 bugs identified as early as 1999 in the system it built," The Guardian wrote in a summary of Patterson's testimony today. The article said:

When bugs were acknowledged, witness statements from Fujitsu staff due to be heard in court were then edited by the Post Office as it sought to maintain the line that the system was working well as it pursued innocent people through the courts.