The connection from male bracket to wall?
The bracket itself?
The female bracket from the shelf?
The connection of the male bracket to the wall will always come undone first, then the bracket itself, then the screws that hold the female bracket in the shelf. The second 2 will never happen. This is because long before that would occur, the screws that held the male bracket to the wall would tear out slowly/gradually - the shelf and its content will already on the floor before the bracket fails or before the female comes undone from the shelf. This is why we cannot stress proper installation of the male bracket into the wall enough.