There is really only 1 way for it to happen. It doesn't actually use cookies. It would happen like this.
Tab 1. You open your preferences page. It generates a key and stores that in the database and puts it into the form. You don't submit.
Tab 2. You open your prefernece page. It generates a key and stores that in the database and puts it into the form. You don't submit.
You go back to tab 1 and submit. The key that is in that form will no longer match the key that is in the database and you'll get that error.
Now why someone would do that, I don't know, but that's how it could happen
