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If the errors are detected by Moz's crawler and Google Search Console (both at the same time) then I'd be much more concerned. It does also depend on the volume of them, if there are like three then it's probably not worth your time to sort it out. If there are hundreds or thousands, you might want to think about that If you have hidden links in the coding which Moz is picking up on (that's how Moz's crawler works, by following links) then you can't really say: "We've checked each page and know that we are not linking to them anywhere on our site" - the fact that the crawler found the links means they exist and are there (even if you can't see them or find them). That is of course, unless your site is on one of the unusual architecture that Rogerbot (Moz's crawler) has difficulties with. That shouldn't be your first assumption, though - he usually knows where he's going Where you say this: "since we migrated our blog to Hubspot so we think it has something to do with the test pages their developers had set up" - pull them up on it! If their developers coded a load of errors into your site, that's their fault not yours and it should be their expense (not yours) to fix it This is the page regarding their CMS: https://www.hubspot.com/products/marketing/content-management-system It does say "A Content Management System Built for Professional Marketers" - so migrating to it, shouldn't cause loads of SEO problems, as SEO is still the largest chunk of most site's online marketing and traffic. That should be nailed down, no problems, fewer problem than your prior system In-fact, HubSpot know that SEO is important for a CMS: https://www.hubspot.com/cms-and-seo - "Every marketer has been told that they need to consider SEO when creating content. But what makes SEO a unique marketing strategy that marketers should prioritize? And why should your CMS have tools that help you execute your SEO strategy?" - I would argue that a load of 404 errors, could not be considered "tools that help you execute your SEO strategy" Whether their developers messed up or their CMS is at fault is not really relevant. The main point is, the responsibility to sort it out should be on their side (not yours, IMO)