Observe the following directory structure:

 index.html
 test/
   bla.txt

And the following configuration:

 DirectoryIndex disabled
 DirectorySlash Off
 FallbackResource /index.html

This all works fine (index.html is shown) if the document can't be mapped to the file system; however, when `/test/` is requested, it will show a 404 page.

This is also disussed in the following question: http://serverfault.com/questions/568952/use-fallbackresource-even-if-directory-exists

I've written a patch for this:

 static int dir_fixups(request_rec *r)
 {
     if (r->finfo.filetype == APR_DIR) {
         /* serve up a directory */
 -        return fixup_dir(r);
 +        if (fixup_dir(r) == DECLINED) {
 +            return fixup_dflt(r);
 +        }
     }
     else if ((r->finfo.filetype == APR_NOFILE) && (r->handler == NULL)) {
         /* No handler and nothing in the filesystem - use fallback */
         return fixup_dflt(r);
     }
     return DECLINED;
 }