What does this mean?
When an error is shown on the page the page title should be updated to display "Error" at the start. When this message is shown this means this is missing.
For example with a page title like this:
<title>What are your contact details? - Service Name - GOV.UK</title>
when the page has an error, the title should show the prefix:
<title>Error: What are your contact details? - Service Name - GOV.UK</title>
Impact on users
The page title is the first thing a screen-reader user hears when the page loads, so it is helpful for them to understand there is something which requires their attention. It also acts as a fallback in case the error summary is not announced on page load.
How to fix
When an error is present on the page ensure the page title is prefixed by "Error:".
If you have alternative languages such as Welsh available you should also check the correct translation is shown.
References
GOV.UK error summary component