Example 10.89.1. Take R = \mathbf{Z}, M = \mathbf{Q}, and consider the family Q_ n = \mathbf{Z}/n for n \geq 1. Then \prod _ n (M \otimes Q_ n) = 0. However there is an injection \mathbf{Q} \to M \otimes (\prod _ n Q_ n) obtained by tensoring the injection \mathbf{Z} \to \prod _ n Q_ n by M, so M \otimes (\prod _ n Q_ n) is nonzero. Thus M \otimes (\prod _ n Q_ n) \to \prod _ n (M \otimes Q_ n) is not injective.
On the other hand, take again R = \mathbf{Z}, M = \mathbf{Q}, and let Q_ n = \mathbf{Z} for n \geq 1. The image of M \otimes (\prod _ n Q_ n) \to \prod _ n (M \otimes Q_ n) = \prod _ n M consists precisely of sequences of the form (a_ n/m)_{n \geq 1} with a_ n \in \mathbf{Z} and m some nonzero integer. Hence the map is not surjective.
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