I have not been able to reproduce it since the first time it happened and I killed all the index.plx process manually.
From http.log
2010:10:26-16:33:44 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:32:01 -0500] "GET / HTTP/1.1" 500 -
2010:10:26-16:33:47 gatekeeper httpd[29274]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:33:47 gatekeeper httpd[29268]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:33:49 gatekeeper httpd[29285]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:33:49 gatekeeper httpd[29288]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:33:50 gatekeeper httpd[29287]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:33:51 gatekeeper httpd[29293]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:34:04 gatekeeper httpd[29292]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:34:09 gatekeeper httpd[29259]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:34:09 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:32:07 -0500] "GET / HTTP/1.1" 500 -
2010:10:26-16:34:15 gatekeeper httpd[29263]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:34:15 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:32:13 -0500] "GET / HTTP/1.1" 500 -
2010:10:26-16:34:15 gatekeeper httpd[29262]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:34:15 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:32:13 -0500] "GET / HTTP/1.1" 500 -
2010:10:26-16:34:17 gatekeeper httpd[29271]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:34:17 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:32:15 -0500] "GET / HTTP/1.1" 500 -
2010:10:26-16:34:38 gatekeeper httpd[29270]: [error] [client 192.168.0.10] Premature end of script headers: index.plx
2010:10:26-16:34:38 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:32:35 -0500] "GET / HTTP/1.1" 500 -
2010:10:26-16:44:39 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:39 -0500] "GET /core/img/topbar/throbber_still.gif HTTP/1.1" 200 304
2010:10:26-16:44:39 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:39 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:44:39 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:39 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:40 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:40 -0500] "POST /index.plx HTTP/1.1" 200 353
2010:10:26-16:44:40 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:40 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:40 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:40 -0500] "POST /index.plx HTTP/1.1" 200 353
2010:10:26-16:44:41 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:40 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:41 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:41 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:44:41 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:41 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:44:41 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:41 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:44:42 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:42 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:42 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:42 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:44:42 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:42 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:43 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:43 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:43 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:43 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:43 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:43 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:44 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:44 -0500] "POST /index.plx HTTP/1.1" 200 353
2010:10:26-16:44:44 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:44 -0500] "POST /index.plx HTTP/1.1" 200 353
2010:10:26-16:44:44 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:44 -0500] "POST /index.plx HTTP/1.1" 200 353
2010:10:26-16:44:44 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:44 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:45 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:45 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:44:45 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:44:45 -0500] "POST /index.plx HTTP/1.1" 200 354
-----------------------------------
2010:10:26-16:48:33 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:33 -0500] "POST /index.plx HTTP/1.1" 200 353
2010:10:26-16:48:33 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:33 -0500] "POST /index.plx HTTP/1.1" 200 356
2010:10:26-16:48:33 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:33 -0500] "POST /index.plx HTTP/1.1" 200 352
2010:10:26-16:48:34 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:34 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:48:34 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:34 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:48:34 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:34 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:48:35 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:35 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:48:35 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:35 -0500] "POST /index.plx HTTP/1.1" 200 356
2010:10:26-16:48:35 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:35 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:48:36 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:36 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:48:36 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:36 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:48:36 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:36 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:48:37 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:37 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:48:37 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:37 -0500] "POST /index.plx HTTP/1.1" 200 354
2010:10:26-16:48:37 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:37 -0500] "POST /index.plx HTTP/1.1" 200 356
2010:10:26-16:48:38 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:38 -0500] "POST /index.plx HTTP/1.1" 200 355
2010:10:26-16:48:38 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:38 -0500] "POST /index.plx HTTP/1.1" 200 332
2010:10:26-16:48:41 gatekeeper httpd: 192.168.0.10 - - [26/Oct/2010:16:48:41 -0500] "GET /core/img/topbar/throbber_animation.gif HTTP/1.1" 200 828
Edit :Screenshots of top also added for further clarification.