If you've noticed an increase in the frequency of "JRun Closed Connection" errors since having migrated to ColdFusion MX 6.1, then you may want to know one factor that is very likely contributing to that increase.

Speaking generally, web requests from the client browser are recieved by the web server and passed to the JRun webserver connector stub. The webserver connector stub communicates with the JRun server on the JNDI port over TCP/IP, typically port 51010. The web request is sent from the connector to the JRun server, and the JRun server will either assign the request to a JVM thread in the running request pool for immediate execution or JRun will assign the request to a thread in queued request pool if the running pool is full.