<p>In geotechnical practice, the mechanical properties of rock are commonly approximated using mean isotropic quasi-elastic moduli. Their measurement is based on the assumption of macroscopic homogeneity of the rock sample. Sedimentary rocks, however, often exhibit a diagenetic layered structure, for example, thin weak layers with preferred orientation. Moreover, even well-polished sample ends may roughen under high loads, increasing friction during testing. The rock structure is usually complex, making experimental investigation of the role of intrinsic irregularities in mechanical measurements difficult, and the end friction under loading is hard to quantify as well. For this reason, a fundamental study was conducted to evaluate the influence of idealized inhomogeneities in the form of thin weak layers with varying orientations on measured Young’s modulus and Poisson’s ratio under different loading regimes. The corresponding experiments were simulated using the Finite Element Method. An important finding is that the mean effective Young’s modulus and Poisson’s ratio were affected differently by the presence of these inhomogeneities. In the lateral loading tests, the standard sample geometry was insufficient to eliminate systematic errors associated with end friction. This study showed that thin-weak layers and end friction at sample ends can noticeably alter measured effective mechanical properties, particularly under lateral loading. For the experimental determination of mechanical moduli under biaxial loading, modified formulas and sample geometries are required, differing from laboratory suggestions for axial loading. This extension of standards represents a practical implication in the measurement methodology.</p>