In plain language
When input is rejected, the error must be identified in text and clearly say which field is wrong.
Who it affects
Everyone submitting forms — especially screen-reader and color-blind users who miss color-only cues.
How to check
Submit forms empty/with bad data: a text message must name each problem field (not just red borders) and be announced to screen readers. Worked example — A podcast app's episode list: submit invalid data and check the error summary: if fields are named ambiguously ('Field 3 invalid') instead of by their actual label, it fails.
Common misconception
A red border on an invalid field is not an error message. This rule requires the error to exist as text that a screen reader can actually discover and read. Real example: on a calendar app's event-creation form, submitting the event form with a missing title turns the field's border red, but no text message says which field failed or why, and screen reader users hear no announcement at all.