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Hydration node mismatch may appear also if you have invalid html, e.g.
<p>
<p></p>
</p>
or
<span>
<p></p>
</span>
In the example above the nesting is illegal, you can’t nest p inside p, or p inside span.
Browser corrects this error that’s where SSR rendered piece of template diverges from what you have finally rendered in the browser.
Source:stackexchange.com