You will get this error if server will loose a client command (client state including move commands and action commands and so on) and will get next command in sequence.
It looks like this: client sends packets, and each packet has it's sequence number. When server receives the packet, it saves this sequence number as LastClientCommand.
When server will loose one packet from sequence of packets, it will know this by doing this: CurrentSequence - LastClientCommand
If the result is 2 or more, then it means that the server lost one or more packets and drops the client with the message you posted above.
Try to run server with certain CVars (developer 2) set so you will get a message like that:
Code:
clientCommand: %s : %i : %s\n
in the place of %i, there will be command sequence numbers
You will probably see a skip if you will manage to reproduce the situation.