A form reports three errors at the top of the page, but the visitor must search the entire form to find them. The summary has identified a problem without providing an efficient route to repair it.
An error summary is most useful when each item names the problem and links to the relevant answer. A message beside the field can then explain the same issue in the place where the correction happens.
Connect the message to the control
The GOV.UK Design System error-summary guidance describes linking summary items to their associated fields and handling focus. Use the documented behaviour of the component you actually implement; a visually similar box does not automatically provide the same accessibility features.
For a single email field, the summary link should take the visitor to that field. For a grouped answer, such as a date split into several inputs, follow the appropriate pattern for the group rather than linking to an unrelated heading.
Keep the summary and inline wording consistent. A summary saying “Enter a date” and an inline message saying “Date is too early” describe different problems. The companion error-message guidance emphasises specific, clear wording that helps someone recover.
Test the correction journey
Submit an intentionally incomplete form using the keyboard. Check where focus moves, what is announced by a screen reader and what happens when each summary link is activated. Then correct one error and submit again.
Preserve valid answers during that process. Requiring someone to re-enter unrelated information makes a small error more expensive to fix. Check the behaviour on a narrow screen and at increased text size as well.
Finally, distinguish user-input errors from a service failure. A server outage cannot be repaired by asking the visitor to change a correctly entered postcode. The design should explain the actual problem and offer the next action that can genuinely help.