There are several recurring 404 errors I see in my 404 error log each month. I investigated some of the more mysterious file names. Perhaps the list below will save other webmasters some research.
This is an information file that shows what FrontPage Server Extensions are installed, their version, and what type of server they're installed on. I assume this file is requested if your web site is accessed using FrontPage. Some sites say this file is requested if your web site is viewed via MS Office.
This is a Microsoft FrontPage file. It is probably requested when your site is viewed using FrontPage.
owssvr.dll (see section "More Information")
This file is used by Microsoft Windows SharePoint Services to process discussion requests. Some sites say it is requested from Internet Explorer when the Discuss Toolbar is turned on.
I found several sites that say this file is requested when a visitor who has MS Office installed uses Internet Explorer to view your site and has the Discuss Toolbar turned on.
A site owner can list his site's main pages in this file. The file is requested by the A9 Toolbar, A9 SiteInfo - Firefox Extension, and any other toolbar that uses the standard. If the file is present the site visitor can click a button on the toolbar and get a drop-down menu of pages on the site.
noexist_[code deleted here].html
I haven't found out anything definite about this one, but it seems to be related to Google Sitemaps. The long code I deleted from the file's name above is a Google Sitemaps verification code.
I don't know what someone or some program is looking for when it seeks 666666, but that file name has been sought in more than one month. It could be a hex color code, an ICQ number, or who knows what.