https://arstechnica.com/tech-policy/2020/10/excel-glitch-may-have-caused-uk-to-underreport-covid-19-cases-by-15841/

Public Health England admitted on Sunday that the agency has under-reported COVID-19 infections by 15,841 cases in recent days due to a "technical issue." The missing positive tests were conducted between September 25 and October 2 and have since been added to national statistics, the agency said.

PHE didn't explain the nature of the technical issue, but a number of British news sources have pointed the finger at Microsoft Excel. Here's how the Guardian describes the issue: