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Flexural and punching failure experiments were performed on flat slabs by controlling the vertical displacement under the column. This allows the monitoring of the descending branch of the load-displacement curve and establishes that flexural failure is characterized by a smooth decrease of the load-carrying capacity, whereas punching failure exhibits a sudden reduction of this capacity. It is shown both experimentally and with force equilibrium that one condition for punching failure that occurs is that the internal strength of the punching cone is larger than the punching strength itself. The punching load was reached for a punching crack inclination of approximately 30 deg (0.5 rad) to the middle plane of the slab. By increasing this inclination, the flexural failure load was approached and the sudden decrease of the load-carrying capacity was reduced. A smooth transition between punching and flexural failure, which is controlled by the punching crack inclination, is revealed, and an interaction formula is proposed.