The 404 error occurs when:
you put a link to a page that doesn't exist, because you have upper / lower case (index or index) is not observed, or a wrong extension (htm or html) specified, or you have simply a mistake.
you used a hyperlink from another website to a non-existing page on our website
you used an "old" search engine link.
a page of our website is registered as a bookmark, but the page does not exist anymore.