http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/273-exit-relay-pinning.html
For example, the first Tor user could be pinned
to exit relay A, the second user could be pinned to exit relay B,
etc. This would allow the website to link together the sessions of
anonymous users.
We cannot prevent websites from serving client-specific policies, but
we can detect it by having Tor Browser fetch a website's ERP policy
over multiple independent exit relays.