For example, User tries to access a.example.com and server presents abc.example.com with a.example.com not in SANs list. Will it be just a warning or an error?
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "Questions about security policies that may be based on cryptography (as opposed to the design of cryptographic algorithms and protocols) are off-topic here, but may be asked on Security." – yyyyyyy, e-sushi
A well designed browser should display a warning page whenever there is a problem with a certificate for the asked paged.
Firefox and Chrome will provide a way to manually bypass the page.