Actually, the story on Williams is a bit more complicated.
First, you have to understand that, while in the U.S. we refer to the ground level as the "first floor" or "first story," and the one above it as the "second floor" or "second story," and so forth in many parts of the world, particularly those with a French heritage, the ground level is referred to as the ground level and the numbering of floors starts with what we call the second floor. The second floor would be the "premier etage," the third floor is the "deuxieme etage," and so forth.
The CI in the Williams case was a Caribbean from Haiti, as I recall. He spoke English poorly, and he referred to the third floor of the building, which was in fact a drug house, as the "deuxieme etage." This was mistranslated by someone at BPD as the "second floor," and the warrant was issued for the second floor.
Williams, a complete innocent, lived on the second floor.
The problem is, of course, that the response of a complete innocent to a sudden invasion of his home by armed men is just about the same as that of druggies: they tend to be defensive and non-compliant with shouted demands to get on the floor.
The solution is for the police to know their neighborhoods and for tactical squads to involve the local beat cops in their operations.