As you set up your site you will need to decide where to put different files and how to organise your directories. There are a couple of directory names you should not use.
Apache is the server level program responsible for displaying webpages. Within the configuration file for Apache, there are a couple of Aliases that it uses for directories.
These are /manual/ and /icons/.
Because of this, you should not use these names for any directories. If you have a file called mypage.html in a directory called /manual and type in http://yourdomain.com/manual/mypage.html then Apache will look it's own directory for mypage.html and since there is no page with that name there it will return a 404 Page Not Found Error. The same would be true for /icons/mypage.html or /icons/myimage.gif.
The solution is to use a different name for these directories, perhaps mymanual or myicons.