The GCC documentation contains a good example:
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

"""
Another use for __builtin_unreachable is following a call a function that never returns but that is not declared __attribute__((noreturn)), as in this example:

void function_that_never_returns (void);

int g (int c)
{
  if (c)
    {
      return 1;
    }
  else
    {
      function_that_never_returns ();
      __builtin_unreachable ();
    }
}
"""

This example is more explicit than a function which gets an "unexpected" value: such case should use Py_FatalError() if I understand correctly.

By the way, Py_FatalError() should be avoided: it's always better to report the failure to the caller :-)