We had a bit of trouble with an Apache2 server, configured as a front-end proxy for a IIS running various different sites. The idea for this has grown because of the fact that our client had only one public IP address at the time, but had to run multiple sites. Rather than running the sites in IIS on different ports, we setup the Apache2 to proxy requests to the IIS back-end, so the sites were accessible by means of sub-urls.
E.g.:

http://myserver/mysubdomain1/ -> http://internal-ip:my-port/

For this, we used mod_proxy, and the ProxyPass and ProxyPassReverse.

Lately, our client complained of regular errors when surfing the sites. Apache2 would give HTTP error 502 (Bad gateway) on the page  ...