With a light httpd worker front end as a reverse proxy to a mod_php backend everything works fine, until I enable APC for PHP on the backend.  If I use wget to make a request to the backend directly, it works fine.  If I restart the httpd backend, it works for one request from the light side, then fails with:
Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /test.php.
Reason: Error reading from remote server
Logs show:
[Sat Apr 07 10:09:02 2012] [error] [client] (70014)End of file found: proxy: error reading status line from remote server
[Sat Apr 07 10:09:02 2012] [error] [client] proxy: Error reading from remote server returned by /test.php
Is this similar to your bug report? http://mail-archives.apache.org/mod_mbox/httpd-dev/201201.mbox/%3C4F25B7B3.6050703@kippdata.de%3E
You can see my full config without the actual vhost here: https://github.com/shazarlx/kloxo/tree/httpd-light_2/kloxo/file/httpd-light
The vhost contains:
        <IfDefine light>
                ProxyPassReverse /
                RewriteEngine on
                RewriteCond   %{REQUEST_URI} .*\.(php)$
                RewriteRule ^/(.*)$1 [P]
Also I've switched the heavy side to prefork from itk but still the same results.