If a user doesn't have a Bitbucket account but they try to log into Bitbucket using the credentials for their Atlassian account, the system will ask them for a username and create a new Bitbucket account for them without further notice. This is creating confusion for users that think they're using the right email address, but they end up with a new empty Bitbucket account instead.
Currently, after they enter the credentials they see "Success! Almost done. Please choose a username for your Bitbucket account." It would be nice to make it clear that a Bitbucket account with that email address does not exist, and we're going to create a new one for them.