404 errors are considered one of the most potential impediments in the way to successful SEO. When a specific URL is renamed or becomes non-existent, any links connecting to that URL would result in 404 errors.

Interesting thing is, Google does not penalize any website for 404 errors. However, if the search engines consistently fail to crawl the internal links of any website, the search ranking of that website is very likely to drop with low traffic.