No worries 
Often it can be triggered after changing the cookie type - if cookies of the other type were previously set it gets itself in a right mess…
I’ve also seen strange session related things happen if you are just logged in on “localhost” (CF10+), or if you happen to start a site on a particular host name, then stop it and run another site on the same hostname and the cookies are still set in the browser from the previous app… Nightmare!