Apart from the fact that the docs to Include do not match the code and IncludeOptional is not documented at all, the way Include interacts with directory wildcards is a bit strange:
I have conf/extra/httpd-userdir.conf and a directory conf/original (without httpd-userdir.conf in it). This makes Include conf/*/httpd-userdir.conf fail with httpd: Syntax error on line 481 of /usr/local/apache2/conf/httpd.conf: Could not open configuration file /usr/local/apache2/conf/original/httpd-userdir.conf: No such file or directory even though the whole Include statement does match one file. I think this makes directory wildcards a lot less useful with Include. Is this intentional or an implementation quirk? Of course, one can always use IncludeOptional... BTW, does anyone have some free cycles to fix the docs?
