How to change the "unknown command" error message with ProtocolLib. Updated for Minecraft 1.6.4.

ChangingUnknownCommand.java

package com.comphenix.example;

import org.bukkit.ChatColor;
import org.bukkit.event.Listener;
import org.bukkit.plugin.java.JavaPlugin;
import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;
import org.json.simple.parser.ParseException;

import com.comphenix.protocol.Packets;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.events.ConnectionSide;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketContainer;
import com.comphenix.protocol.events.PacketEvent;

public class ChangingUnknownCommand extends JavaPlugin implements Listener {
    @Override
    public void onEnable() {
        ProtocolLibrary.getProtocolManager().addPacketListener(
            new PacketAdapter(this, ConnectionSide.SERVER_SIDE, Packets.Server.CHAT) {
                private JSONParser parser = new JSONParser();

                @SuppressWarnings("unchecked")
                @Override
                public void onPacketSending(PacketEvent event) {
                    try {
                        PacketContainer packet = event.getPacket();
                        String json = packet.getStrings().read(0);

                        JSONObject data = (JSONObject) parser.parse(json);
                        String message = ChatColor.stripColor((String) data.get("text"));

                        if ("Unknown command. Type \"/help\" for help.".equals(message)) {
                            // Change the output
                            data.put("text", "Wrong. Try again!");
                            
                            // Write back the resulting JSON
                            packet.getStrings().write(0, data.toJSONString());
                        }
                    } catch (ParseException e) {
                        throw new RuntimeException("Cannot parse JSON.", e);
                    }
                }
            });
    }
}

Use this version for 1.7.2:
https://gist.github.com/aadnk/8129389