An error message that only says 'invalid input' is not helpful

An error message that only says 'invalid input' is not helpful. It is a dead end.

Actual users know something went wrong. What they do not know is where, why, or how to fix it without starting over.

WCAG in plain English: identify the field in error, describe the problem in text, and suggest a correction when one is known. Do not rely on color alone. Do not clear the form. Do not move focus somewhere confusing and hope people find their way back.

We test it this way: submit an incomplete form on purpose, with a keyboard and with a screen reader. Can someone find every error, understand each one, fix it, and resubmit without re-entering good data?

If not, the error handling is not finished.

Clear errors are not polish. They are how a service respects people's time.