Canonical error, broken links error, site structure, navigation, keyword stuffing, internal linking are some errors to make a site rank down.
Normally a page with an error appears when you try to visit a page of a website that doesn't exist anymore. The page doesn't exist in the url that you are trying to visit whether because there is a broken link whether because you made changes in the navigation or site structure. I hope that helps you
there are many errors on the website such as designing error, website content error, HTML editing error, site map creation error, linking error, etc.
There are some free websites out there that are going to give you results for almost every error on your website whether its code error also called syntax errors, or stylesheet or deadlinks..... I am posting 2 of them for now.... http://www.websiteoptimization.com/services/analyze/ http://www.pearanalytics.com/ To find more in google yourself with this keyword "free website analysis tool"....