This counts IGMP packets received where the Layer 2, Ethernet-level, multicast MAC address doesn't correctly correspond to the Layer 3, IP-level, multicast group address it's supposed to represent, per the standard IP-to-Ethernet multicast MAC mapping convention. An admin seeing this counter increment should suspect either a misbehaving multicast source generating incorrectly-mapped frames, or possibly an attempt to exploit the many-to-one nature of the IP-to-MAC multicast mapping to reach unintended groups. A rising count correlating with a specific new source device coming online would point to that device's multicast frame generation as the likely culprit.