This paper is devoted to the numerical simulation of dynamic crack propagation using X-FEM and level set techniques. The paper proposes an explanation of three experiments. The first experiment shows the dynamic propagation of a crack that avoids a hole placed on its trajectory. The second presents the interpretation of experiments of two crack tips which run in the same direction, and the third studies the interaction between two cracks which move toward each other. It is shown in all the cases that the crack tip avoids the other discontinuity (hole or other crack tip). An explanation is proposed through the observation of transient stress fields. Copyright © 2011 John Wiley & Sons, Ltd.