I was on https://circleci.com/gh/mediapop/patontheback-mobile/edit#checkout clicked “Authorize with Github” did the whole OAuth flow got sent from https://github.com/login/oauth/authorize to https://circleci.com/gh/mediapop/patontheback-mobile/edit#checkout which gave me a 404 because I got logged out.
I had to go back to the page, auth again for it to work.