Sometimes you will get error codes from the server.
Here is a small documentation of those error codes.
You sent to many packets per second.
Your packet was larger than 1024 bytes.
You didn't send a move packet.
Your ip is already connected.
You didn't send the join packet.
Your first packet was not a join.
You used an invalid username.
You used a forbidden username.
Your Username is too short.
Your Username is too long.
You used an invalid password.
Your Password is too short.
Your Password is too long.
You don't have permission to do this.
Your password is wrong.
You are already connected with this player.
You tried to do something illegal :O
You did not send a valid move packet.
You sent an unknown command.