Bell nonlocality is a remarkable nonclassical property of correlations between certain quantum systems. Under some simple assumptions it rules out the existence of hidden variables that predetermine the outcomes of measurements on the systems, and forms the basis for quantum information protocols that promise the secure distribution of cryptographic keys and the generation of guaranteed random numbers. However, the promise of Bell nonlocality and such information protocols requires a particular assumption: that the choice of measurement made on each system is uncorrelated with other physical variables.
It is in fact possible to build (and sell!) physical devices that violate this assumption, allowing an adversary to have full knowledge of the measurement outcomes (and cryptographic key or "random" numbers) generated by the devices. Optimally efficient models of such devices, in terms of the information cost required to subvert Bell nonlocality, require only ~0.08 bits of information transfer from the source to the measurement devices on each run. The practical moral is to source one's equipment from more than one supplier! Further, if retrocausal or backwards-in-time information transfer is allowed, then just ~0.04 bits of information transfer are required. This latter result is of theoretical interest in providing a type of Ockham's Razor argument for retrocausal explanations of Bell nonlocality.