What's wrong

This problem is specific to errors that happen on the client after the initial mounting of the component. (The router has special logic to handle exceptions thrown in getInitialProps during a client-side navigation, and I've confirmed this logic is correct.)

Specifically, if the page is mounted, and you raise an exception on the page, the exception will cause the error page to be mounted without ever invoking getInitialProps on the new App/Error page pairing.

This has been illustrated with multiple repros in #4574.

Why is it broken

This regression was introduced two months ago in #4156, where the invocation of getInitialProps was removed from the app's top-level error handler. Specifically, this line was removed and replaced by a comment that says that "App will handle the calling of getInitialProps".

I believe the sentiment about "App will handle calling getInitialProps" is mistaken. In fact, it really doesn't make sense on its face, since it would require an instance lifecycle method of App (which is mounted immediately after the comment) to invoke the static getInitialProps method on the error page.

How I fixed it

I've fixed this in a fork by restoring Lines 146 – 148 that were removed in #4156. I think this is the right fix, but Next.js's handling of getInitialProps could certainly be improved. (The code in this conditional speaks to the unnecessary complexity around this.)