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Anomalous behavior is investigated of differential resistance dU/dI of laser diodes at the lasing threshold. The regular case is the known decrease of the dU/dI associated with a dynamic saturation of the junction voltage. In contrast to this, the anomalous case is an increase of the differential resistance (the "positive kink" of dU/dI). Some regular samples are found to show the anomaly at lower temperature or at high hydrostatic pressure. The anomaly is discussed in terms of the injection-induced conductivity.