There are many ways the web server encounter 404 error. Maybe you leave old link around, or a single typo will redirect your site visitor to a non-exist page, or the typo is made by the visitor when typing your website URL, and many more reason.
If your site using WordPress, then you can create a custom 404 error page. You can put a more informative text, or maybe auto redirect the user to home page. Check out the latest article in 1st webdesigner that specially talk about how to create a custom error 404 page for wordpress. At the end of the article, there are 10 example of creative 404 page for your inspiration.
