Steps to reproduce:
- Create a totally new account on the English Wikipedia.
- Go to https://tools.wmflabs.org/oauth-hello-world/
- Click "Authorize this application"
Expected result: I am able to authorise the application
Actual result: Error "Unified login needed, E008"
The error is caused because the totally new user doesn't have a local account on mediawiki.org. One is created immediately afterwards, but it's that little bit too late. If you refresh the page showing you the error, it works just fine.
See also T94885