A formula for the commutator of tensor product matrices is used to show that, for qubits, compatibility of quantum multiparty observables almost never implies local compatibility at each site, and to predict when this happens/does not happen in a concise manner. In particular, it is shown that two fully nontrivial-qubit observables are compatible locally and globally if and only if they are equal up to sign. In addition, the formula gives insight into the construction of paradoxes of the type of the Kochen-Specker theorem, which can then be easily rephrased into proposals for no-hidden-variable experiments of the type of the Bell theorem without inequalities.
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