It is of course super common. Unit tests should be stubbing out as much as possible/reasonable. The presumption is that we're talking about a well rounded test

It is of course super common. Unit tests should be stubbing out as much as possible/reasonable. The presumption is that we're talking about a well rounded test suite with both unit tests and integration tests, and the integration tests would indeed be catching the fact that A is passing entirely the wrong thing to B. Of course, this does mean we're leaning heavily on the test suite here. But, in any nontrivial application I hope that we would have a robust test suite, right? Otherwise we're going to have some problems whether we're statically or dynamically typed.