Mention <title> and styled-jsx caveats in Document's docs (#11499)

* Mention <title> and styled-jsx caveats * Update custom-document.md Co-authored-by: Tim Neutkens <[email protected]>
vercel · Apr 6, 2020 · 969ffd6 · 969ffd6
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diff --git a/docs/advanced-features/custom-document.md b/docs/advanced-features/custom-document.md
@@ -53,6 +53,7 @@ The `ctx` object is equivalent to the one received in [`getInitialProps`](/docs/
 - React components outside of `<Main />` will not be initialized by the browser. Do _not_ add application logic here. If you need shared components in all your pages (like a menu or a toolbar), take a look at the [`App`](/docs/advanced-features/custom-app.md) component instead
 - `Document`'s `getInitialProps` function is not called during client-side transitions, nor when a page is [statically optimized](/docs/advanced-features/automatic-static-optimization.md)
 - Make sure to check if `ctx.req` / `ctx.res` are defined in `getInitialProps`. Those variables will be `undefined` when a page is being statically exported by [Automatic Static Optimization](/docs/advanced-features/automatic-static-optimization.md) or by [`next export`](/docs/advanced-features/static-html-export.md)
+- Common errors include adding a `<title>` in the `<Head />` tag or using `styled-jsx`. These should be avoided in `pages/_document.js` as they lead to unexpected behavior
 
 ## Customizing `renderPage`