What are the causes of HTTP 404 errors?

HTTP 404 error is technically a client-side error which means it is your mistake, i.e., the requested page is not present in your website. If you had maintained that page in your site, it would have been indexed by the crawler and thus would have been present in the server. Furthermore, you also receive this error, when you mistype a URL or when a webpage or resource is moved without redirecting the old URL to the new one. So, whenever you move your webpage redirect the old URL to the new URL to avoid this error as it may affect the SEO of your site.