This counter tallies IGMP packets whose Layer 2 Ethernet multicast destination MAC address does not correctly correspond to the Layer 3 IP multicast group address the packet claims to carry, per the standard IP-to-MAC mapping rule. On this switch such a mismatch typically points to a misbehaving host stack, a NAT/tunnel device rewriting only one of the two addresses, or a spoofing attempt, so any sustained count is worth tracing back to the offending source MAC on that port. As an example, a badly configured video encoder that rewrites its source IP without updating its outgoing multicast MAC could push this counter up by dozens of packets during a channel change.