In a recent paper, Mishra et al. [Effect of obstacle strength and spacing on the slope of Haasen plot. Mater Sci Technol. 2019;1–6. doi:10.1080/02670836.2019.1567043], discrepancy between theoretical and experimental results in the slope of the Haasen plot between pure metals and alloys were reported. The discrepancy arises from a common belief that when Haasen plot for pure metals is extended to metals with multiple strengthening mechanisms, the inverse of activation volume components can be linearly superposed. It is conjectured that the slope of the Haasen plot should remain the same for both the pure metals and alloys, as it is governed only by dislocation–dislocation interaction. The purpose of this note is to clarify that the conjecture of invariant Haasen slope is only a special case. © 2019, © 2019 Institute of Materials, Minerals and Mining.