Funny that https://stackoverflow.com/ certificate is not valid.
or... have i been working on this bug for way to long?
They need to be serving a different cert on a different IP for that (ignoring SNI since not all browsers support it).
However, Chrome surprised me the other day by not allowing me to continue on to a site that didn't have a valid certificate (I was MiTM myself using PAROS proxy to test something). In other words, we don't need stricter warnings, but the warnings must become errors for them to be noticed.