  In his paper "Residues of a Pfaff system relative to an invariant subscheme"
in Trans. Amer. Math. Soc. 352, 2000, 4019-4035, F. Sancho de Salas defines the
universal Koszul complex of a module $M$ over a sheaf of rings $\mathcal{O}$ as
${\rm Kos}(M)=\Lambda (M)\otimes_{\mathcal{O}}S(M)$, where $\Lambda (M)$ and
$S(M)$ stand for the exterior and symmetric algebras of $M$, endowed with the
usual differential, and he conjectures (Conjecture 2.3.) that ${\rm Kos}(M)$ is
always acyclic. We give here an example of a non acyclic Koszul complex ${\rm
Kos}(M)$.
