See PHI2165.
A repository may have a remote URI specified like turtle@example.com:/path/to/repository.git, and be configured to use a credential with a different explicit username like duckling.
Currently, this resolves into an invocation of ssh -i identity.key -l duckling -- turtle@, which is ambiguous, and different versions of SSH might possibly have different behavior in the face of this ambiguity (PHI2165 reports a behavioral change which is possibly, but not conclusively, a result of an SSH version upgrade).
This should be disambiguated in the UI, probably by warning that the URI username will become controlling and choosing it over the credential username (since this is strictly more flexible in all cases I can come up with).