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is PHP configured on your server to run as an apache module or in CGI mode?Server API: Apache 2.0 Handler
RewriteRule \.htm/ NonexistentPageName.htm
For quite some time I've been seeing 404 errors in Google WMT that would come from existing pages being artificially put into existing folders they don't belong to.
WMT reports back 404 for /sub/page1.html based on the link from /sub/page2.html/a-b-c where a-b-c is a string created from the title of the page.
Don't use relative linking to images, CSS files or JS files. Start the href with a leading slash and include the full path to the file. Your site is then impervious to crawl errors such as this when using rewrites or AcceptPathInfo.
How is WMT getting the idea that these pages "should" exist?Don't know yet. I started looking into raw log files. Google WMT does not help as I can only see the dates when the server started returning 404 for those links. I actually need to see the initial request whenever it happened. The rest was just an effect of broken linking.
How (and where) is the title-of-the-page string being created and appended to the URL?No idea. This may take another investigative effort like this AcceptPathInfo which BTW has been mentioned here many times. I'll be searching more on this through old WebmasterWorld posts.
Don't use relative linking to images, CSS files or JS files.This does not matter as such pages do not exist nor they should return 200, at least in this case. These pages have to return 404.