For a tensor named T with two indices, there are four possibilities: $T_{ij}$ , $T_i^{\ j}$, $T^i{\ _j}$ and $T^{ij}$. Is there a common convention as to how these tensors would be represented as matrices, i.e. where the entries would go? Is it the left-right order of the indices that determines which matrix entry is meant, or some other convention? What if the the order of the indices in a mixed tensor is not indicated at all (as in $T_i^j$)? Is it true that, for instance, the component with i=2 and j=3 would go on the second row and the third column in all of the above cases? The books will just say "$F_{μν}$ = [some matrix]", and you don't know which is which.
Below is an example that is in itself contradictory. To convey the idea that F is antisymmetric, they use two different conventions in the very same line - here it is the order of the Greek subscripts that determines the order.
$$ F_{\mu \nu} = \left( \begin{array}{cccc} 0 & -E_1 & -E_2 & -E_3 \\ E_1 & 0 & B_3 & -B_2 \\ E_2 & -B_3 & 0 & B_1 \\ E_3 & B_2 & -B_1 & 0 \end{array} \right) = -F_{\nu \mu} $$