Stable usually doesn't get such issues. They are related to transitions. Sometimes it happens, that some packages are stuck in unstable for example, because they don't build on some arch, while their related packages go through. In result, testing gets an inconsistent combination, while unstable is OK. It happens when someone didn't take care to specify that these packages should only move to testing together.

This won't happen with stable, since it will get the consistent result in the end.