Interference between the different mass eigenstate components of a neutral K meson causes its decay probability to oscillate with time. Related oscillations occur in the decay chain [phi] → KK → f1f2 (where f{sub 1,2} are decay channels), in neutral B decay, in the chain [Upsilon](4s) → BB → f1F2, and in massive neutrino propagation. Since the mass eigenstates comprising a neutral K, a neutral B, or a neutrino have different masses, they have different speeds at any given momentum. Thus, classically, they become separated in space and time. This circumstance can tempt one to evaluate their contributions to the K or B decay, or to the neutrino interaction with a detector, at different spacetime points. However, these quantum-mechanically interfering contributions must always be evaluated at precisely the same point. Evaluating them at different points can lead to predicted oscillation frequencies double their true values.