I think you should have an unit test with the current behaviour and in the comments, add the right test and right behaviour. Example:

    public void test() {
      // this is wrong, it should be fixed some time
      Assert.assertEquals(2, new Calculator().plus(2,2));
      // this is the expected behaviour, replace the above test when the fix is available
      // Assert.assertEquals(4, new Calculator().plus(2, 2));
This way, when the fix is available, the build will fail, noticing you the failed test. When you'll look at the test, you will know that you changed the behaviour and the test must be updated.