In a case like this, you really ought to have a standard set of unit tests that covers the contract of the interface. If you run that against all the implement

In a case like this, you really ought to have a standard set of unit tests that covers the contract of the interface. If you run that against all the implementations and none of the tests fail, the bug just isn't in the implementation itself and you can safely move on to looking for integration issues. Of course, if the unit tests for the interface are incomplete you can't rely on them, but in that case you've built yourself a leaky abstraction that you should go back and look at before doing any more systems testing. Effective dependency injection simply requires comprehensive unit testing.