Cisco's Output Interpreter gives this:
%C4K_EBM-4-HOSTFLAPPING (x1):Host [mac-addr] in vlan [dec] is flapping between
port [char] and port [char]
Explanation: The specified host is detected as a source address on multiple ports.
Typically, a host is supposed to be learned on only one port. A spanning tree
loop is the most common cause of this condition. All traffic from the specified
host is temporarily dropped. After 15 seconds, forwarding is reenabled. If the
problem persists, the traffic from the problem host continues to be dropped indefinitely.
Recommended Action: Make sure that spanning tree is enabled to prevent spanning
tree loops. If you have spanning tree disabled, make sure that you have no spanning
tree loops in your network.
