This work gives new insight into coalescence in protein-stabilized emulsions, based on a detailed experimental study of highly concentrated protein-stabilized emulsions. These highly concentrated emulsions are shown to release large quantities of oil in a coalescence process if they are deformed by an external stress, provided the volume fraction of emulsion droplets is above a critical value. During the coalescence process, macroscopic fracture planes are formed, through which the oil separates. On the basis of these observations and rheological measurements, a new mechanism is proposed in which friction between the droplet surfaces plays a dominant role. In this mechanism, the volume fraction of the emulsion droplets is related to a capillary pressure pushing the droplets together. The capillary pressure produces a pressure normal to the thin films, pushing the two film surfaces together. This increases the frictional coupling between the two surfaces. Below the critical volume fraction, friction is relatively low and the surfaces may slip. However, above the critical volume fraction, the frictional forces become so high that they cannot be supported by surface tension differences across the droplet surfaces without destabilizing the thin film. Moreover, because stresses around a ruptured film cannot relax by slip between the droplet surfaces (reorganization of the droplets), film rupture can propagate along stress planes in the emulsion. This explains the experimental observation of formation of fracture planes through which liquid oil separates, which is typical for this coalescence process.