Many Covid cases were initially missed or under-reported due to... spreadsheets and file size limits:

This glitch meant that 15,841 cases between September 25 and October 2 were left out of the reported daily coronavirus cases.

The issue occurred because some files containing positive test results exceeded the maximum file size that takes these data files and loads then into central systems, officials said.

They said that “rapid mitigation” measures have been put in place to ensure the issue does not occur again.

But the blunder will have led to an inevitable delay in some contacts being reached.

Source: https://www.theargus.co.uk/news/national/18770299.data-blunder-affect-battle-covid-19/