Leading-edge cavitation is responsible of the generation of transient cavities, usually made of clouds of bubbles. These transient cavities travel downstream to high-pressure regions and collapse violently, leading to noise and vibration as well as erosion. In the present paper, the focus is on the mechanisms generating transient cavities to better understand the starting point of the erosion process. The case studied is the cavitating flow over a NACA0009 hydrofoil which is investigated using experiments and numerical simulation. In the experimental part, which is conducted in EPFL high-speed cavitation tunnel, the shedding behavior is studied using high-speed visualization (HSV). In the numerical part, the cavitating flow is simulated using an incompressible solver coupled with isothermal homogeneous two-phase mixture cavitation model and Implicit Large Eddy Simulation (ILES) turbulence modelling. Owing to high speed visualization and numerical simulations, we identified two shedding mechanisms of transient cavities: (i) A primary shedding, characterized by a periodic generation of large cloud cavities and (ii) a secondary shedding of small-scale horse-shoe vortices, which are revealed for the first time. These small-scale structures, which are believed to play a major role in the erosion process, result from a complex interaction between the sheet cavity, the cloud cavity and re-entrant jets of different types. Furthermore, the detailed comparison between HSV and simulation confirms that the current numerical approach is capable of capturing the two types of shedding mechanisms very well. (C) 2019 Elsevier Ltd. All rights reserved.