I logged in to Stack Overflow with Stack Exchange OpenID using an old email account. Now, even after removing cookies in FireFox, etc, I can't log in with my other account. Every time I click "Log in with Stack Exchange", the email field is never displayed to allow me to enter my other account email, it just logs me in with the account I don't want, which is this one.
There are three accounts in question here: your Stack Overflow account, your global Stack Exchange account, and your Stack Exchange OpenID account.
When you click "Log In with Stack Exchange", it actually means "Log In with a Stack Exchange OpenID account". If you create an account from the login page, you're actually creating an account on and logging in to http://openid.stackexchange.com/, despite the fact that you never see this site.
Because of various compatibility issues with cross-domain cookies, Stack Exchange OpenID accounts and global Stack Exchange accounts store your login credentials using your browser's LocalStorage instead. These are less straightforward to erase.
Using the "Log Out" page on Stack Overflow will log you out of Stack Overflow account and your global Stack Exchange account, but it will not log you out of your Stack Exchange OpenID account. You must also go to http://openid.stackexchange.com/ and click the "Log Out" button there.
The technical pieces are reasonable, but as a user experience this is absurd and dangerously confusing. I have no doubt that countless users have clicked "Log Out" on Stack Overflow, the only one of these sites they've ever visited, and had no clue, nor even any reason to suspect, that their authentication was still active.
Stack Exchange sites keeps on tracking your IP Address. Once you login from a computer with some IP address say 18.104.22.1688 . This IP address is saved in the Stack Exchange's system. Now whenever you try to login from the same system with some other account, the system will logs you in with the older account
I also encounter the same problem. First I logged in with Google chrome and then I tried to log in with some other account through Mozilla Firefox, but the system logs me in with the older account that I used in Google Chrome.