I'm in Gmail with 3 Google accounts signed-in. When I choose to switch to a different user account from the drop-down menu in the upper right of Gmail, it pops open a new Gmail window for the same user as before. What gives?
I meet same problem many times and trust its authorisation scripts problem. Google try to make accounts switching easy - just this brings some small problems with so many services they offering using same authorisation data.
I use iGoogle which loads for specific account and sometimes it ask me to re-enter account password to be able to bring me mail messages inside iGoogle. In other words to access my iGoogle account the password is ok, but NOT to access the same account mail messages. Authorisation scripts used in iGoogle and on Gmail gadget on it are different authorising on different ways - but trust they write in same variables in my browser to keep my login data for all further cases when account authorisation will be required.
Every account have own number in the list of all available accounts inside current browser.
The primary account have id 0, then all other have own number depending of the order of signups. When you switch google accounts the account id is passed through URL:
If this not works the sign-out is the only available option, which must clear all authorisation problems.
This seems to happen from time to time in my experience (4 accounts), especially if I try to log into two accounts at once from the same domain (examples: firstname.lastname@example.org and email@example.com)
Just restarting the browser works much of the time.
If not, logging out and starting over usually works. Starting over = logging into each account again.
Rare cases I have to log out and wait 30 minutes to try again.
To log into another account from the same domain use the generic GMail login screen and enter the full email address as the user id.