On June 19, 2003 server security was increased and one of the side effects is that some special characters are no longer allowed in Web document filenames or folders. You may not use the ampersand ( & ), percent ( % ) or the period ( . ) characters or as part of a name for HTML pages, graphics, Word documents or any other file or folder. If you do, a "404 File Not Found" error will display.
While these are the specific characters involved, be aware that others such as the space, slash ( / ) and dollar ( $ ) should not be used. Just stick with letters and numbers and you should not experience problems. The dash ( - ) and underscore ( _ ) are allowed, however.
See Naming Files & Understanding URLs for more information.