Ask them to write the tests that will trigger the failure condition - i.e. make them prove that it's a possible thing that can easily occur. If it is, then at

Ask them to write the tests that will trigger the failure condition - i.e. make them prove that it's a possible thing that can easily occur. If it is, then at least you have a simple test case to work with so they've done a chunk of the work for you. If it's not, then they'll spend ages trying to craft a test case for a scenario that's extremely unlikely. People very often leave you alone when challenged to put in the work to prove their point.