When trying to browse https://stackoverflow.com I receive an error that the certificate is not valid. The reason is that *.stackoverflow.com comes with a certificate for *.stackexchange.com at the moment.
This is a race condition as we upgraded certs immediately but DNS took a bit to catch up. You should now be seeing a valid cert which we'll be updating again as soon as we receive a new combined wildcard for that particular certificate.
Most CAs are a bit busy today which is understandable.