The Only Time You'd Notice a Black Belt Is Having a Bad Day

The only time you'd notice he's having a bad day is if he's fighting against another person who's very high level. So in general, when we're talking about it in general, it means weak points. If he fights a high level person, you might see his chinks in the armor. And then you'll see where he's messing up to mess up means to do something wrong to make a mistake.