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Google's John Mueller said that if you implement structured data on a page, that structured data should match the content visible on your page. The most obvious example is if you add review markup to your page but there are not reviews visible to the end user, that would be spammy structured markup.
Here is John's tweet:
You don't have to put SD on all pages of a site, but if you're going to put SD on a page, it should match the visible content.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) November 20, 2017
Of course, there can be some gray areas, which is why Google has some pretty detailed guidelines for when and how to implement structured data on your site.
Follow those guidelines or you can lose the ability for your site to show rich snippets in the Google search results.
Forum discussion at Twitter.
