A function call necessitates setting up the stack for the call, and returning from the call by popping values of the stack. To get 100% coverage, you'll have to test the call, test the broken call, test the stack being corrupted, and so on. This adds to the cyclomatic complexity of the TESTING required to certify that your software doesn't have/take any unknown paths during its lifetime while installed in highly hostile environments (heat, radiation, water, desert conditions, Antarctica, etc.)