If a user creates a new account on en.wiki (and most other wikis, I assume) and then tries to log in to an OAuth app that authenticates on that same wiki, they will not be able to do so until they visit another wiki while logged in.
- Register a new account on en.wikipedia.org
- Go to wizard.wikiedu.org and attempt to log in
- You will get the E008 error.
- Go to wizard.wikiedu.org again (any number of times, even after waiting days or weeks) and attempt to log in.
- You will get the same error.
- Visit another wiki while logged in (such as commons.wikimedia.org), then try again.
- Now you can log in successfully.