Find the pair of test cases causing a failure

You have a black-box function called run() that takes a list of test cases as input. It is known that there are two specific test cases that cause the run to fail only when both of them are present in the input list; otherwise, it will pass. Find these two test cases. Example Input testcases = ['A', 'B', 'C', 'D'] Output Problematic pair: ['B', 'C']