An equation is written as $\mathbf{x} = \mathbf{y} + z$. In the notation system being used, bold letters (like $\mathbf{x}$ and $\mathbf{y}$) represent quantities with multiple components, while italic letters (like $z$) represent quantities with a single component. A core principle of this system is that addition is only defined between quantities of the same type (i.e., multi-component can only be added to multi-component). Identify the typographical error in the equation and explain why it creates a logical inconsistency based on the given principle.