In an interesting conversation on Twitter, Google’s Gary Illyes has confirmed that 404 errors will not lead to Google penalties for your website. 404 errors occur when a web page has been removed from your website or does not exist. Barry Schwartz from Search Engine Roundtable explained that there is a rumor out there that “having a site with some 404ed pages will lead to a Google Penalty.”. Google has now confirmed this.
— Barry Schwartz (@rustybrick) August 7, 2015
I agree that just the existence of 404 errors on your website is not going to lead to a penalty or manual action from Google. There are perfectly legitimate reasons to have 404 errors on your website. For example, if someone is linking to a page on your website with a lot of bad links, then you can move the location of that page to another URL and serve up a 404 error. That will essentially ‘get rid of’ those bad links and not pass them onto the rest of your website. But, that’s all great if those bad links are linking to an internal page on your site: most negative SEO and low quality links I’ve seen tend to be pointed towards a site’s home page.
404s won’t lead to a penalty, but it’s not good for your site overall.
Technically speaking, 404 errors on your website won’t get you a penalty from Google. But, let’s look at it another way: if your internal linking structure is messed up and your site is linking internally to pages that don’t exist, then you’re shooting yourself in the foot. Your 404 errors are causing problems, and you’re also going to cause crawl issues.
So while 404s are not technically going to get you a penalty, you’re just shooting yourself in the foot by not fixing them.
The goes for duplicate content on your website, as well. Unless you have so much of it you’re spamming the search engines, you’re just not going to typically get a penalty from Google just because you have duplicate content on your website.