Displayed matrix works:
$\begin{bmatrix}
a & b \\ b & c
\end{bmatrix}$

Non-displayed matrix does not work due to some backslash escaping we do not want touching tex:
$\begin{bmatrix}
a & b \\ b & c
\end{bmatrix}$

Make $cash

Proof that this is the problem:
$\begin{bmatrix}
a & b \\\\ b & c
\end{bmatrix}$