  The aim of this short Note is to show that the Schr\"odinger and Heisenberg
pictures of quantum mechanics are not equivalent unless one uses a quantization
rule clearly stated by Born and Jordan in their famous 1925 paper. This rule is
sufficient and necessary to ensure energy conservation in Heisenberg's matrix
mechanics. It follows, in particular, that Schr\"odinger and Heisenberg
mechanics are not equivalent if one quantizes observables using the Weyl
prescription.
