An analog of Fano's theorem for ionization in cavities is shown to hold for the stepwise representation of electron paths used in Monte Carlo computer models of electron transport. This brings to light an error in the distribution of electron paths and hence energy deposition which is induced by interrupting steps which cross the interface between media of different densities. The magnitude of the error depends on the shape of the cavity and its size relative to the electron path length in the cavity gas. In a typical calculation of a cylindrical chamber exposed to 60Co radiation, if the electron step size is taken as 10% of the remaining path, then a 3% energy deficit in the cavity results. An algorithm for crossing an interface is described which does not produce this error.