For security purposes it is not allowed for users to login multiple instances of their account. This will result in an error like the image below.
An example would be :
A user logs in on their account using Chrome. The user needed to test something in IE so he opens his IE and logs into his account. After his testing is done, he goes back to Chrome and tries to click on a Tab. He will get the error defined in this article since NetSuite will identify that his most recent session is not on Chrome.
To resolve this, a user need to close the session in IE or just simply close the IE browser and login to Chrome again to restart session.