Take $A=\begin{bmatrix}1&2&0\\-1&0&3\end{bmatrix}$ and $B=\begin{bmatrix}2\\1\\4\end{bmatrix}$. Their shapes are $2\times3$ and $3\times1$, so $AB=\begin{bmatrix}4\\10\end{bmatrix}$ has shape $2\times1$. The reverse product $BA$ is undefined because its inner dimensions do not match.
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