= Solution
The indices have mixed parity, so the face-centred lattice sum vanishes: \b[the ideal NaCl $(300)$ reflection is systematically absent]. A smaller wavelength, a larger detector range or an extra rotation axis does not remove that extinction. A nominal peak there could reflect another wavelength/reflection, multiple diffraction, another phase or broken ideal translational symmetry; it would not be an ordinary kinematic $(300)$ reflection of ideal NaCl. The allowed $(600)$ peak is a different reciprocal-lattice vector, not a recovered $(300)$ peak.
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