Advancing the knowledge about the punching behaviour of flat slabs has been held back by the fact that only indirect measurements of the internal failure mechanism have typically been available. In this paper, a novel experimental approach for tracking the formation and development of cracks inside punching test specimens is presented. In six full-scale specimens without shear reinforcement, 48–64 measurement points were installed inside small holes drilled on the soffits of the slabs. During the punching tests, the displacements of these points were followed at various levels of load with a high-precision coordinate measuring arm. Development of inclined flexural cracks (called the critical shear cracks) from the tension chord to the neutral axis was observed in every specimen. In some cases, the punching failures occurred along these cracks. However, in other cases, new lower-angled failure cracks developed, either at the moment of failure or already at earlier stages of loading. These cracks had very different patterns of development, even between two monitored sides of the same specimen, although the specimens were nominally axis-symmetric.