The Include directive, when given a directory as parameter (such as /etc/proftpd/conf.d/, as in the stock /etc/proftpd/proftpd.conf), causes all files in said directory to be read, not just ones ending in .conf. This causes problems if, for instance, I’m using vim to edit a file in the included directory while the proftpd service is being (re)started: vim stores a .swp file in the same directory, and proftpd may fail to start with ”fatal: unknown configuration directive” when it tries to parse the .swp file.