This counter tracks frames that failed to transmit because they encountered a collision outside the normal collision window. Late collisions are a classic symptom of a duplex mismatch or an Ethernet segment that's physically too long for the configured speed, distinguishing this fault from the 'heavy traffic' cause associated with the max-collisions counter. Seeing even a handful of late collisions on a point-to-point management link, which should never experience real collisions at all, is a strong hint that one end is set to full-duplex while the other is stuck at half-duplex.