When a repository is configured for --ff-only merges, if a cross-repository pull request fails to merge because it's not fast-forward, the error message is useless if the fork's branch name matches the target repository's branch name:

KEY/slug is configured to require fast-forward merges, but master includes commits which have not been merged to master. To perform this merge, master must first be merged into master.

In a same-repository pull request this couldn't happen because both branches must have different names. But when merging in changes from a fork, those changes can be applied on a branch with the same name as the destination being targeted upstream.

When performing a cross-repository merge, the source branch should be qualified with the project key and repository slug for the source repository.