Wheeeee, this was a hard one to track down... Essentially this is a long standing bug related to ad caching that was exaggerated by another bug in the newsletter ad rendering path. It manifested itself when there is a race between anonymous and registered users.
Ads are heavily cached and the cache key is constructed based upon a number of parameters. One of those parameters is whether you are subscribed to a newsletter. Registered users can either be subscribed or not subscribed and anonymous users are always not subscribed. In the backend we had checks for anonymous users that weren't respected in the rendering code.
That meant that when an anonymous user visited the site (and the cache was not primed) the HTML would incorrectly contain the newsletter ad and it would subsequently be cached under the key for unsubscribed users. When an unsubscribed registered user came along it simply rendered the previously cached HTML.
In the rare case that an unsubscribed registered user came to the site (and the cache was not primed) then the correct output would be displayed.
That HTML also had a URL to a specific user - the user that hit it with an unprimed cache. Clicking subscribe would attempt to subscribe using that users identifier. Fortunately the backend prevents such shenanigans - hence the error message.
Fun, fun, fun... fix is deployed to production now!