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Mechanisms that make dislocation motion irreversible are associated with the formation of dislocation junctions and cross-slip, leaving dislocations trapped inside the specimen. Using Discrete Dislocation Dynamics simulations, we identify another mechanism that produces irreversible plastic deformation and leaves no or only very few dislocations inside the sample: Under cyclic loading, dislocations which pass the neutral plane during loading (pile-up formation), generate a slip step upon unloading. The explanation is an intrinsic asymmetry between the backward and forward motion. An additional bias may be introduced by the geometry of the specimen due to the shortening of the line length of dislocations.