If you try to edit a redlink or go directly to the URL for editing a non-existent page, it throws a fatal error and sends you to a 404 page.
For example, try going to:
Same behavior in alpha, beta, and stable. (You can follow redlinks from lazy-loaded pages after editing even in stable.)
It looks like it is trying to send the user to Special:MobileEditor/<pagetitle>, which causes a fatal error and then redirects to:
This bug is currently blocking reproduction of VE bug 67639.