"not first!" WARN log message

Hi,

I am seeing the following log in my ZK (version: 3.4.5),

WARN org.apache.zookeeper.server.quorum.Leader - Commiting zxid
0x100000000 from {ip address:port} not first!

I have no idea what causes it and where to start debugging. The
closest I could get was the ZOOKEEPER-1270.

Does anyone know what causes ZK to log the above message and possible
solutions as well.

Thanks.

Manoj