Login form and back button

[eluser]gyo[/eluser]
This is a common issue which has nothing to do with CodeIgniter or the Sessions class.
Basically when you hit the back button on the browser, it gives you the cached page without re-asking it to the server.
It's supposed to work like that.

Try to search for "back button" here in the forum to get more info about it.
Also try loggin to whatever website (amazon, ebay, facebook) and you'll notice the same "issue".