the access_log contains errors like:
10.16.89.42 - - [24/May/2011:13:54:52 -0500] "GET /st/SessionTestServlet? HTTP/1.1" 500 608 "-" "Jakarta Commons-HttpClient/3.1"
error_log in debug shows the corresponding error:
+++
[Tue May 24 13:54:52 2011] [debug] mod_proxy_cluster.c(1245): proxy: byrequests balancer DONE (ajp://10.16.88.180:8009)
[Tue May 24 13:54:52 2011] [debug] mod_proxy.c(1015): Running scheme balancer handler (attempt 0)
[Tue May 24 13:54:52 2011] [debug] mod_proxy_http.c(1955): proxy: HTTP: declining URL ajp://10.16.88.180:8009/st/SessionTestServlet?
[Tue May 24 13:54:52 2011] [debug] mod_proxy_ajp.c(668): proxy: AJP: serving URL ajp://10.16.88.180:8009/st/SessionTestServlet?
[Tue May 24 13:54:52 2011] [debug] proxy_util.c(2011): proxy: AJP: has acquired connection for (10.16.88.180)
[Tue May 24 13:54:52 2011] [debug] proxy_util.c(2067): proxy: connecting ajp://10.16.88.180:8009/st/SessionTestServlet? to 10.16.88.180:8009
[Tue May 24 13:54:52 2011] [debug] proxy_util.c(2193): proxy: connected /st/SessionTestServlet? to 10.16.88.180:8009
[Tue May 24 13:54:52 2011] [debug] ajp_utils.c(31): Into ajp_handle_cping_cpong
[Tue May 24 13:54:52 2011] [debug] ajp_utils.c(102): ajp_handle_cping_cpong: Done
[Tue May 24 13:54:52 2011] [debug] ajp_header.c(224): Into ajp_marshal_into_msgb
[Tue May 24 13:54:52 2011] [debug] ajp_header.c(290): ajp_marshal_into_msgb: Header[0] [User-Agent] = [Jakarta Commons-HttpClient/3.1]
[Tue May 24 13:54:52 2011] [debug] ajp_header.c(290): ajp_marshal_into_msgb: Header[1] [Host] = [dev66:8888]
[Tue May 24 13:54:52 2011] [debug] ajp_header.c(450): ajp_marshal_into_msgb: Done
[Tue May 24 13:54:52 2011] [debug] mod_proxy_ajp.c(265): proxy: APR_BUCKET_IS_EOS
[Tue May 24 13:54:52 2011] [debug] mod_proxy_ajp.c(270): proxy: data to read (max 8186 at 4)
[Tue May 24 13:54:52 2011] [debug] mod_proxy_ajp.c(285): proxy: got 0 bytes of data
[Tue May 24 13:54:52 2011] [debug] ajp_header.c(687): ajp_read_header: ajp_ilink_received 09
[Tue May 24 13:54:52 2011] [debug] ajp_header.c(697): ajp_parse_type: got 09
[Tue May 24 13:54:52 2011] [debug] mod_proxy_ajp.c(555): proxy: Processing of request failed backend: 1, output: 0
[Tue May 24 13:54:52 2011] [error] proxy: dialog to 10.16.88.180:8009 (10.16.88.180) failed
+++

In fact it is due to failing cping/cpong a lot earlier:
+++
[Tue May 24 13:52:30 2011] [error] ajp_cping_cpong: apr_socket_recv failed
[Tue May 24 13:52:35 2011] [error] ajp_cping_cpong: apr_socket_recv failed
+++