I had registered my account through Google (AFAIK it was on Stack Overflow, but don't really remember), and I'm logged in on Stack Overflow. However, when I went to chat.stackexchange.com domain (including any of the chatrooms), I wasn't logged in. When I attempted to log in using my Google account it said the user with that email doesn't exist and it offered to create a new account.
Logging in on Super User did make everything fine and automatically logged me across SE as well (including the chat), but it also left me wondering what would've happened if I proceeded with the sign up with the already existing email.