You use an invalid or customized client.
If it's on a live server, the dude either tried to craft and send a packet (using L2PHX) or tried to connect using an invalid/customized client.
Shortly said, a packet was sent (generally by client) which was denied by gameserver which define which packet is supposed to be triggered. It avoids packets to be triggered manually, on unwanted times (eg a packet craft using connection packet during ingame would probably lead to issues).
Some anticheat systems can also generate new packets, leading to that type of error.