Quassel does not warn if SSL certificate changed on remote IRC server (not core)
One of the IRC servers I used upgraded to a new system and in the process changed their SSL certificate. All that happened is that I didn't automatically reconnect. Once I made the connection manually, it just went with no warning that the cert had changed.
While I'm not sure how important it is to do cert CA validation, I do think it's very important that if the cert for a channel changes the user is warned before reconnecting. This is important to prevent MITM attacks.