1. A method of processing requests from clients, comprising: at one or more servers: receiving requests from clients and sending responses, at least a subset of the responses including cookies produced by the one or more servers; at least a subset of the received requests including cookies that were previously produced by the one or more servers and sent to the clients with responses to previously processed requests; each of the cookies including a respective user identifier; analyzing the received cookies, comprising cookies included in the received requests from the clients, to detect a first condition, wherein the first condition indicates that a respective plurality of distinct clients may be using cookies that have a same shared user identifier; upon detecting the first condition, sending at least one of the clients of the respective plurality of distinct clients a response that includes a new cookie having a user identifier that is distinct from the shared user identifier; analyzing the received cookies to detect a second condition, the second condition indicating that a respective client has repeatedly failed for a predetermined number of times to replace an old cookie with a new cookie sent by any of the one or more servers to the client; and upon detecting the second condition, performing a first predefined remedial action; wherein the received cookies comprise first cookies and the received requests comprise first requests; and wherein performing the predefined remedial action includes sending the respective client a response that includes an executable reference, wherein the executable reference is configured so that execution of the executable reference by the respective client would send a second request with a second cookie distinct from the first cookies from the respective client to a respective server.