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Flysch rock masses pose significant challenges in geotechnical design due to their pronounced heterogeneity, anisotropy, and susceptibility to disintegration, particularly in deep urban excavations. These characteristics often result in unpredictable slope behavior, requiring a careful combination of geotechnical modelling and practical experience.This study presents a case of an excavation approximately 11 m high, constructed for a building in the Pujanke area of Split, Croatia, situated in typical Dalmatian flysch. The original design proposed a wide excavation with relatively gentle slopes, based on conservative geotechnical parameters previously validated in similar conditions through the authors’ prior studies. During construction, the designed geometry was not fully respected, and significantly steeper slopes were implemented, leading to localized slope failure. This situation provided a rare opportunity to observe classical disintegration mechanisms, layer interactions, and the influence of flysch heterogeneity on excavation stability, complementing insights from previous research.Following the collapse, a remediation project was successfully implemented. However, due to subsequent modifications of the excavation geometry and construction conditions, additional design iterations were required on the same slope. These successive redesigns illustrate the core of rework in AEC design, where changes in fundamental assumptions such as geometry, boundary conditions, and construction phasing necessitate repeated reinterpretation of the same geotechnical problem.A back-analysis of slope stability demonstrated a strong correspondence between previously proposed design parameters and the actual behaviour of the rock mass, confirming their appropriateness and highlighting the critical importance of strict adherence to design assumptions during execution. The study further discusses various technical solutions and their robustness against potential deviations from planned conditions, including minor slope modifications and reinforcement measures.The results contribute to a better understanding of the behaviour of flysch rock masses in deep excavations and provide practical guidance for safer and more resilient geotechnical design in urban areas with heterogeneous soft rocks, enabling a more stable continuity of design assumptions and a reduction in rework in AEC design.