See [1] on why this is happening. Relevant part:
>Internally, the web server associates a token called the fnid with each instance of a link or form, and records the continuation function for each token. Periodically, old fnids are deleted.
>Internally, the web server associates a token called the fnid with each instance of a link or form, and records the continuation function for each token. Periodically, old fnids are deleted.
Any idea where the fnids are stored and how they are garbage-collected? Could it be that they're just dumped into a memcached instance and older fnids get pushed out whenever the memory limit is reached?