Earthing and lightning protection systems are critical to the safety and reliability of electrical networks, and their performance must be validated through field testing. The Fall-of-Potential (FoP) test method is one of the most common techniques for measuring earth grid resistance, valued for its simplicity and practical relevance. In the field, however, non-uniform soil conditions such as fill material, moisture changes, or layered geology often distort potential gradients and produce resistance curves that deviate from the expected 62% rule. If these results are accepted without considering the soil environment, engineers risk drawing incorrect conclusions, leading to unnecessary remedial works or underestimation of system hazards. This presentation draws on case studies where layered soils produced non-traditional FoP curves, showing how correlating measurements with local soil data restored confidence in the results. Practical strategies such as adjusting probe spacing, relocating test lines, and incorporating soil resistivity data are also discussed to improve reliability. By emphasising engineering judgement and field awareness, the presentation highlights how FoP testing can be applied more effectively in real-world environments to support safer and more robust earthing systems.