Program: 86053
Project: 86053
Buggy source:
@Override public void onPluginMessageReceived(String channel,Player player,byte[] message){
  if (!channel.equals("String_Node_Str")) {
    return;
  }
  ByteArrayDataInput in=ByteStreams.newDataInput(message);
  String subchannel=in.readUTF();
  System.out.println(subchannel);
  if (subchannel.equals("String_Node_Str")) {
    short len=in.readShort();
    byte[] msgbytes=new byte[len];
    in.readFully(msgbytes);
    DataInputStream msgin=new DataInputStream(new ByteArrayInputStream(msgbytes));
    try {
      final String playerData=msgin.readUTF();
      final String plugin_=playerData.split("String_Node_Str")[0];
      final String arena=playerData.split("String_Node_Str")[1];
      final String playername=playerData.split("String_Node_Str")[2];
      System.out.println(plugin_ + "String_Node_Str" + arena);
      JavaPlugin plugin=null;
      for (      JavaPlugin pl : this.pinstances.keySet()) {
        if (pl.getName().contains(plugin_)) {
          plugin=pl;
          break;
        }
      }
      if (plugin != null) {
        final Arena a=pinstances.get(plugin).getArenaByName(arena);
        if (a.getArenaState() != ArenaState.INGAME && a.getArenaState() != ArenaState.RESTARTING && !a.containsPlayer(playername)) {
          Bukkit.getScheduler().runTaskLater(this,new Runnable(){
            public void run(){
              if (!a.containsPlayer(playername)) {
                a.joinPlayerLobby(playername);
              }
            }
          }
,20L);
        }
      }
    }
 catch (    IOException e) {
      e.printStackTrace();
    }
  }
}

Fixed version (ground truth):
@Override public void onPluginMessageReceived(String channel,Player player,byte[] message){
  if (!channel.equals("String_Node_Str")) {
    return;
  }
  ByteArrayDataInput in=ByteStreams.newDataInput(message);
  String subchannel=in.readUTF();
  System.out.println(subchannel);
  if (subchannel.equals("String_Node_Str")) {
    short len=in.readShort();
    byte[] msgbytes=new byte[len];
    in.readFully(msgbytes);
    DataInputStream msgin=new DataInputStream(new ByteArrayInputStream(msgbytes));
    try {
      final String playerData=msgin.readUTF();
      final String plugin_=playerData.split("String_Node_Str")[0];
      final String arena=playerData.split("String_Node_Str")[1];
      final String playername=playerData.split("String_Node_Str")[2];
      System.out.println(plugin_ + "String_Node_Str" + arena);
      JavaPlugin plugin=null;
      for (      JavaPlugin pl : this.pinstances.keySet()) {
        if (pl.getName().contains(plugin_)) {
          plugin=pl;
          break;
        }
      }
      if (plugin != null) {
        final Arena a=pinstances.get(plugin).getArenaByName(arena);
        if (a.getArenaState() != ArenaState.INGAME && a.getArenaState() != ArenaState.RESTARTING && !a.containsPlayer(playername)) {
          Bukkit.getScheduler().runTaskLater(this,new Runnable(){
            public void run(){
              if (!a.containsPlayer(playername)) {
                a.joinPlayerLobby(playername);
              }
            }
          }
,20L);
        }
      }
    }
 catch (    IOException e) {
      e.printStackTrace();
    }
  }
 else   if (subchannel.equals("String_Node_Str")) {
    short len=in.readShort();
    byte[] msgbytes=new byte[len];
    in.readFully(msgbytes);
    DataInputStream msgin=new DataInputStream(new ByteArrayInputStream(msgbytes));
    try {
      final String requestData=msgin.readUTF();
      final String plugin_=requestData.split("String_Node_Str")[0];
      final String arena=requestData.split("String_Node_Str")[1];
      System.out.println(plugin_ + "String_Node_Str" + arena);
      for (      JavaPlugin pl : this.pinstances.keySet()) {
        if (pl.getName().contains(plugin_)) {
          Arena a=pinstances.get(pl).getArenaByName(arena);
          if (a != null) {
            BungeeUtil.sendSignUpdateRequest(pl,pl.getName(),a);
          }
          break;
        }
      }
    }
 catch (    IOException e) {
      e.printStackTrace();
    }
  }
}