why occured the 404 error in several websites
404 means page not found, or a typo in your URL. If you want to know something else, please provide more information.
It is an error message for not found information. As said by pritaeas, post something for more information.
404s are definetly not good for SEO in cases where a page has moved. If this is your scenario you need to have 301/302 redirects in place so the web spiders can follow.
404 error is a code that tells page not found
It’s page not found error. When visitor page is not available on server.
This message appears when there is some problem on server or that page is not found on server.
404 Page Error is Page Not Found. this website page not Creating.
when web page not work and broke on that time the webpage show 404 error.