Recently Evert Pot has been having some issues with setting up lighttpd and PHP (fastcgi) for one of his web servers:

In trying to get more out of our webservers using a Lighttpd and PHP-FastCGI setup, I've come across some major issues that make it difficult to use. I hope this post will warn people of some of the bugs they might encounter and workaround that might need to be implemented until some of these are fixed.

He ran up against two problems - that the parent PHP-CGI process spawns off the configured number of children correctly but with the lighttpd server is killed, they don't get released and that, once you hit full capacity for the PHP backend lighttpd starts serving up 500 errors.

In the comments for the post several others have offered some advice on things that might help (like different config settings and even using XCache to prevent the 500s issue from coming up again).