Noticed this while reviewing https://github.com/vercel/next.js/pull/14376. After having done https://github.com/vercel/next.js/pull/13699, this code didn't feel right to me:
```js
function prepareRoute(path: string) {
  path = delBasePath(path || '')
  // this /index rewrite is problematic, it makes pages/index.js 
  // and pages/index/index.js point to the same thing:
  return toRoute(!path || path === '/' ? '/index' : path)
}
```
Added a nested index page to the prerender tests and found it was rendering the `/` route on navigation. This uncovered 2 more places around the dataroute where the index path was not translated correctly.

**edit:**

Just to note that there was nothing wrong with https://github.com/vercel/next.js/pull/14376, the issue was already there, I just noticed it while reading that PR

Fix https://github.com/vercel/next.js/issues/14290 and a couple other issues around optional catch-all that popped up after writing these tests

Closes https://github.com/vercel/next.js/pull/14344

In serverless mode, it's best practice to propagate an unhandled error so that the function is disposed instead of recycled. This helps fix the "bad state" problem.

This addresses some errors for `/_next/data` requests where encoded `/` values in dynamic route param would cause invalid behavior, a headers already sent error would be shown when sending the fallback page in development, and when rendering the `_error` page for a data request the error response would still be treated as a data request. 

This also adds test cases for these errors to prevent regression

Since non-fallback pages don't rely on the URL for hydration we can allow them to be rewritten to but pages with fallback still can't be rewritten to because we won't be able to parse the correct `/_next/data` path to request the page's data from. I added a test for this behavior and ensured it works correctly on Now.

Example on with fallback false rewrite on Now:
https://tst-rewrite-cp9vge4bg.now.sh/about

* Rename unstable GSP revalidate field

* Update error message

* Tweak error message some more

* Apply suggestions from code review
Co-Authored-By: NJoe Haddad <joe.haddad@zeit.co>
Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

* Verify GS(S)P Serializability

* Add support for cyclic refs

* Add unit tests

* Test for error in development mode

* Fix prerender preview tests

* Fix gssp preview tests

* fix 2x test cases

* Add desired test

* fix some more tests

* Fix route manifest expect

* Fix test expects

* Test that `getServerSideProps` does not error in production

* Test that getStaticProps is not checked in production

* Test serialization check during build

* Fix export detection for serverless

* Update test/unit/is-serializable-props.test.js
Co-Authored-By: NJJ Kasper <jj@jjsweb.site>
Co-authored-by: NJJ Kasper <jj@jjsweb.site>

* Fix Prerender Test Cases

* fix test

* Rename getServerProps to getServerSideProps

* Remove unstable_ prefix from new methods

* Add error when legacy methods are detected

* Add legacy methods for babel transform

* Add unstable_getServerSideProps also

* Apply suggestions from code review
Co-Authored-By: NJoe Haddad <joe.haddad@zeit.co>

* Update types import
Co-authored-by: NJoe Haddad <timer150@gmail.com>

* Ability to Disable SSG Fallback

* Throw error when value is missing

* Fix existing tests

* Adjust error message

* Do not render fallback at build time for `fallback: false` page

* Fix existing fallback behavior

* fix build

* fix version

* fix some tests

* Fix last test

* Add docs for get static paths

* Add explicit mode tests

* test for fallback error message

* Improve Nested Catch-All Coverage

* use check

This adds a `isFallback` property to detect if the page is being rendered in "fallback" mode or normal mode.
Accessed via the `useRouter()` hook.

---

Closes #10527

* Add paths field for unstable_getStaticPaths

* Make sure to specify page in getStaticPaths errors

* Add initial SSG fallback handling

* Remove extra changes and update fallback handling

* Remove extra timeout for testing

* Update SSG tests in dynamic-routing suite

* Add racing to decide between rendering fallback and data

* Update size-limit test

* Update comment

* Make sure to follow correct route change order

* Make comment more verbose for racing

* Revert getStaticData to only return Promise

* Make sure to update URL on fallback

* Add retrying for data, de-dupe initial fallback request, and merge fallback replace

* Update to add preload for fallback pages data

* Add test for data preload link

* Use pre-built fallback in production mode

* Remove preload link for fallback from _document

* Update to make sure fallback is rendered correctly for serverless

* De-dupe paths returned in getStaticPaths

* Remove warning

* Add support for catchall routes with SSG

* Add test for invalid catchall param in getStaticPaths

* Fix SSG data request failing with trailing slash

* Update to re-use toRoute method

* Add failing test for re-calling getStaticProps after updating query

* Fix get(Static|Initial)Props re-running when updating query

* Update invalid export tests
Co-authored-by: NJoe Haddad <timer150@gmail.com>

* Use Cached SSG Data on History Navigation

* Add data caching test

* Create a static data cache

* Eliminate an if / return

* Do not cache in dev mode

* bump

* bump

* bump

* bump
Co-authored-by: NJJ Kasper <jj@jjsweb.site>

* Fix query values being passed in dev mode for SSG

* Update test names

* Improve Build Output for SSG

* Swap out symbols and reasons

* adjust text more

* Add test

* Expect more output lines

* fix test case

* Adjust return value

* Add delay

* Remove Extraneous Imports in Pages

* Add test

* bump cache key

* Make sure to parse query values for SSG page after hydration

* Update to re-use __NEXT_SPR

* WIP: rename get static paths

* Adjust logic

* Add warning

* rename constant

* Check for error when using old method

* Add friendly error message

* Test for message of invalid keys

* replace index with dots

* Update `packages/`

* Update examples

* Update tests

* Update bench

* Update top level files

* Fix build

* trigger

* Disallow iSSG revalidation period of zero

* Fix revalidation period in test

* Add handling for exportTrailingSlash for iSSG pages

* Update index.test.js

* Apply format to webpack config

* hit all files

* Ensure directory before flushing cache iSSG file

* Add test for prerender cache flush

* Nest the dynamic route test one more level

* update fetch for test

* Update error check

* Don't mark page as nextExport if it's SPR

* Add test for fetching prerender data after mount

* Configure the Default Revalidate
The default revalidate behavior should be configured by Next.js. Otherwise, the behavior might drift or change in non-semver compliant ways between Next.js and the builder (or other 3rd party setups).

* Add additional comment