More info: I tried recent stable version (0.7.62) and it shows the same behavior--everyone gets the query. Replaced mongrel servers with webrick, and the problem disappears. Now, one request gets sent to one backend, as it should. FWIW, the mongrel version is 1.1.3 Posted at Nginx Forum: http://forum.nginx.org/read.php?2,10652,10844#msg-10844