This counter tallies Ethernet frames sized between 256 and 511 bytes (counting FCS octets, excluding framing bits) that were received with errors across all ports in a group, the errored counterpart of the matching Ok-sized bin. A nonzero and growing value in this specific 256-511-byte error bin points to a physical-layer problem affecting frames of roughly that size on the group's shared wiring. An admin comparing this counter against gsobkt256to511Ok on group 4 could estimate what fraction of 256-511-byte traffic on that microhub is arriving damaged.