I'm a belt-n-braces coder, and I believe it is perfectly reasonable to have a test in one part of the code that will never be triggered unless at some point in the future another part of the code changes. But it is notoriously difficult to test that all possible error paths are correctly handled.

Consider whether it might make sense to split the routine into two - some methodologies would in any case consider it bad form to have a function that performs two operations. This would allow you separately to test that the first routine cleans the data, and that the second routine correctly barfs on bad data.

Hugo