I have faced strange problem recently. In arp table of active hsrp router (cisco WS-C6509-E (R7000)) there was unknown mac-address found(which was overwriting mac-address of the firewall within the vlan). On the standby router everything was ok. Moreover, that specific mac address couldn't be found within mac-address table of the active or standby switch (aggregation layer). Temporary work around has been applied by setting static arp entry on both routers. However we still cannot define how is that possible.