In what scenario does a client application display a certificate warning to the user?

A client application displays a certificate warning to the user primarily when the certificate's subject name does not match the URL. This situation indicates a potential security risk, as the application is being presented with a certificate for a different domain than the one the user is trying to connect to. This mismatch can be a sign of a man-in-the-middle attack or other forms of internet fraud, where an attacker attempts to impersonate a legitimate website.

When users see a warning related to this discrepancy, it serves as an alert that they should proceed with caution. This helps maintain trust in secure communications, as users are made aware that the connection they are trying to establish does not correspond to the identity that the SSL/TLS certificate claims to represent.

While the other scenarios like an expired certificate, an untrusted certificate authority, or a self-signed certificate indeed warrant caution and often lead to warnings, the specific condition of a subject name mismatch is particularly critical, as it suggests a direct risk to the authenticity of the website in question.