Just a couple of days ago I've run into a tricky bug with nginx. One of the virtual servers defined a variable in its configuration (via set keyword). When creating a new virtual host by copying the site file I've removed the set line, but not the usage (fastcgi_param line). This caused a random sequence of bytes to be put into that variable. PHP serving those FCGI requests handled it gracefully, but I spent quite a long time trying to figure out why this variable was garbled.