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The unreliability of diffuse-reflexion recordings obtained from oblique sections of axially extended diffuse-reflecting regions in reciprocal space is demonstrated by reference to a commonly used diffraction geometry with which the <100> anomalous `spike' X-ray reflexions of natural diamond are studied. A comparison between a sharp `spike' pattern (produced by a region of the crystal containing relatively large platelet precipitates on { 100}) and a more diffuse `spike' pattern (produced by a region containing smaller platelets) shows how the combination of spike diffuseness with fairly high obliquity in sectioning of the spike by the Ewald sphere produces marked discrepancy between apparent and predicted positions of the maxima of the more diffuse `spikes'.