How to handle InvalidAuthenticityToken

Scenario:

1. Log in a user.
2. Go to a page with a form.
3. Open a new tab and log out the user.
4. Go back to previous tab and submit form.
5. It throws InvalidAuthenticityToken before getting to my login code.
I need to redirect the user to a log in screen. What's the best way to
capture this and handle this?

If anyone else is running into this issue here is how I ended up
handling it

http://gist.github.com/94801