A person fills a long request form, presses save and sees “invalid input.” Their entries disappear. The application has detected a problem but made it harder to resolve.
Explain the affected field
Use wording that names the issue and the expected correction. Keep valid entries where appropriate so the person does not repeat the whole task. A summary and a link to the field can help on longer forms.
The W3C form validation tutorial provides guidance on accessible validation and feedback. Checks in the browser support the user, while the server still needs to enforce the application's rules.
Test ordinary mistakes
Include missing required information, an invalid date, unusually long text and a connection failure. Check keyboard access and how errors are announced by relevant assistive tools.
Distinguish a correctable input error from an uncertain save outcome. If the connection drops after submission, a retry should not accidentally create another request.
Ask users to recover from a mistake during prototype review. Their experience helps show whether the form supports the task in practice, beyond looking tidy when every field is filled correctly.
Check the first invalid field on a phone. A visible summary at the top is useful only if the person can reach and understand the correction without losing their place.