Ran into this error when adding a new route.
Turns out that I was stuffing the
model.errors into the alert flash, making the session cookie relatively huge.
While stuffing all that data into the cookie wasn't intended, it made me notice that we were storing the session in a cookie instead of a database like with other projects. It turns out that the ActiveRecord session store has been extracted out of Rails into its own gem recently, the details of which are available on the activerecord-session_store GitHub project page.