During the public prerelease period of a version of Windows some time ago, I remember a bug came in from an application developer. It went something like this:

Hi, we just tried our program on the most recent public prerelease version of Windows, and we noticed that you changed the undocumented XYZ function. Our program uses that undocumented function, and your change to the function breaks us. can you change it back? Yes, we understand that there are no guarantees surrounding the use of undocumented APIs, but still we’d like you to continue supporting the old version. It should just be a few dozen lines of code on your side.

How very nice of the application vendor to identify their compatibility issues for us. Now we don’t need to create a compatibility shim, because not only do we know that the application vendor is still actively supporting the program in question, but that they have already been alerted to the problem with plenty of advance warning for them to issue a patch to their customers before the next version of Windows is released.

The product team rejected this change request with the simple comment, “Unsupported APIs are unsupported.” Presumably the customer liaison translated this into something a bit more polite before communicating it back to the vendor.