duplex mismatch
termOne end at full duplex, the other at half: the link works, slowly and with late collisions - the classic 'network is weird' root cause of the 2000s.
Usually born from one side hard-coded and the other autonegotiating; the autoneg side falls back to half.
The mechanism is auto-negotiation's fallback. A port set to negotiate that hears nothing from a fixed-speed neighbour uses parallel detection, which can sense the speed from the signal but, in the University of New Hampshire lab's words, 'can never parallel detect to a full duplex link', so it assumes half duplex. Fix one end at 100 full, leave the other on auto, and you have built the mismatch on purpose.
The symptoms split by side. The half-duplex end sees late collisions and a 'rapid increase of FCS and alignment errors', because the full-duplex end transmits without waiting its turn; the full-duplex end counts runts. The link stays up and small tests pass, so the fault hides until load arrives.
The rule is about pairs: both ends on auto-negotiation, or both fixed to the same values. Cisco's advice is to leave auto-negotiation on for devices compliant with 802.3u, and from 1000BASE-T on, negotiation is mandatory anyway.