Next.js Beginner: Why Am I Facing Hydration Mismatch Errors with Server-Side Rendering?

Hi everyone! I'm pretty new to the Next.js ecosystem and absolutely loving the potential, especially for building performant web apps. I'm trying to get a handle on server-side rendering for some dynamic content, but I've hit a frustrating roadblock that I just can't seem to solve.

The main issue I'm encountering is a "Hydration Mismatch" error in my browser console on the initial page load. It usually appears as:

Warning: Prop `className` did not match. Server: "..." Client: "..."

or sometimes a more general mismatch. This happens specifically when I'm trying to fetch data using getServerSideProps and then render it.

Here's a quick rundown of my setup and what I'm trying to do:

• I'm using Next.js 13.5.x with the App Router (though this specific issue is in a pages directory setup for now).
• My page component uses getServerSideProps to fetch some data (e.g., a list of items from an external API).
• The fetched data is then passed as props to the main component for rendering.
• The component renders a simple list or card layout based on this data.

I've tried a few things to debug this, but nothing seems to stick:

• I've tried wrapping parts of my component in a useEffect hook with an empty dependency array to ensure they only render client-side after hydration, but that feels like it defeats the purpose of SSR.
• I've double-checked that the data types returned by getServerSideProps are consistent with what the component expects.
• I've simplified my component drastically to rule out complex JSX or conditional rendering issues.
• I've made sure there are no browser extensions interfering.

My expectation is that the content rendered by the server should perfectly match what the client hydrates, leading to a flicker-free experience. Instead, I get this warning and sometimes a slight visual jump.

Could anyone please shed some light on common pitfalls for beginners using server-side rendering with getServerSideProps that lead to hydration errors? Are there specific debugging techniques I should be using? Any best practices for handling dynamic data during SSR?

Anyone faced this before?