Program: 51741
Project: 51741
Buggy source:
public void interpret(String message){
  if (message.startsWith("String_Node_Str")) {
    message=message.substring(1,message.length());
  }
  String[] parts=message.split("String_Node_Str");
  Client client=this.activity.getCurrentChannel().getClient();
  Server server=this.activity.getCurrentChannel().getServer();
  String command=parts[0];
  if ((command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) && parts.length > 1) {
    String[] channels=parts[1].split("String_Node_Str");
    String[] passwords=null;
    if (parts.length > 2 && parts[2].split("String_Node_Str").length == channels.length) {
      passwords=parts[2].split("String_Node_Str");
    }
    for (int i=0; i < channels.length; i++) {
      if (!this.looksLikeChannel(channels[i])) {
        this.activity.getCurrentChannel().addError(SpannedString.valueOf("String_Node_Str" + channels[i]));
        this.service.activeChannelMessageReceived(this.activity.getCurrentChannel(),null);
        continue;
      }
      if (passwords != null) {
        client.joinChannel(channels[i],passwords[i]);
      }
 else {
        client.joinChannel(channels[i]);
      }
    }
  }
 else   if (command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) {
    if (parts.length == 1) {
      client.partChannel(this.activity.getCurrentChannel().getChannelInfo());
      this.service.channelParted(this.activity.getCurrentChannel(),client.getNick());
    }
 else {
      if (!this.looksLikeChannel(parts[1])) {
        client.partChannel(this.activity.getCurrentChannel().getChannelInfo(),parts[1]);
        this.service.channelParted(this.activity.getCurrentChannel(),client.getNick());
      }
 else {
        if (parts.length < 3) {
          Channel channel=server.getChannel(parts[1]);
          client.partChannel(channel.getChannelInfo());
          this.service.channelParted(channel,client.getNick());
        }
 else {
          Channel channel=server.getChannel(parts[1]);
          client.partChannel(channel.getChannelInfo(),parts[2]);
          this.service.channelParted(channel,client.getNick());
        }
      }
    }
  }
 else   if ((command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) && parts.length > 1) {
    if (!client.userExists(parts[1])) {
      client.changeNick(parts[1]);
    }
 else {
      this.activity.getCurrentChannel().addError(SpannedString.valueOf("String_Node_Str" + parts[1]));
      this.service.activeChannelMessageReceived(this.activity.getCurrentChannel(),null);
    }
  }
 else   if ((command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) && parts.length > 1) {
    for (int i=1; i < parts.length; i++) {
      client.sendRawLine("String_Node_Str" + parts[i]);
    }
  }
 else   if (command.equalsIgnoreCase("String_Node_Str") && parts.length > 1) {
    Channel channel;
    if (this.looksLikeChannel(parts[1])) {
      channel=server.getChannel(parts[1]);
    }
 else {
      channel=this.activity.getCurrentChannel();
    }
    client.setMode(channel.getChannelInfo(),message.substring("String_Node_Str".length() + 1,message.length()));
  }
 else   if (command.equalsIgnoreCase("String_Node_Str") && parts.length > 2) {
    if (this.looksLikeChannel(parts[1])) {
      client.sendCTCPCommand(server.getChannel(parts[1]).getChannelInfo(),parts[2]);
    }
 else {
      client.sendCTCPCommand(parts[1],parts[2]);
    }
  }
 else   if ((command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) && parts.length > 1) {
    client.sendRawLine(message.substring(parts[0].length() + 1,message.length()));
  }
}

Fixed version (ground truth):
public void interpret(String message){
  if (message.startsWith("String_Node_Str")) {
    message=message.substring(1,message.length());
  }
  String[] parts=message.split("String_Node_Str");
  Client client=this.activity.getCurrentChannel().getClient();
  Server server=this.activity.getCurrentChannel().getServer();
  String command=parts[0];
  if ((command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) && parts.length > 1) {
    String[] channels=parts[1].split("String_Node_Str");
    String[] passwords=null;
    if (parts.length > 2 && parts[2].split("String_Node_Str").length == channels.length) {
      passwords=parts[2].split("String_Node_Str");
    }
    for (int i=0; i < channels.length; i++) {
      if (!this.looksLikeChannel(channels[i])) {
        this.service.activeChannelMessageReceived(this.activity.getCurrentChannel(),Channel.createError(SpannedString.valueOf("String_Node_Str" + channels[i])));
        continue;
      }
      if (passwords != null) {
        client.joinChannel(channels[i],passwords[i]);
      }
 else {
        client.joinChannel(channels[i]);
      }
    }
  }
 else   if (command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) {
    if (parts.length == 1) {
      client.partChannel(this.activity.getCurrentChannel().getChannelInfo());
      this.service.channelParted(this.activity.getCurrentChannel(),client.getNick());
    }
 else {
      if (!this.looksLikeChannel(parts[1])) {
        client.partChannel(this.activity.getCurrentChannel().getChannelInfo(),parts[1]);
        this.service.channelParted(this.activity.getCurrentChannel(),client.getNick());
      }
 else {
        if (parts.length < 3) {
          Channel channel=server.getChannel(parts[1]);
          client.partChannel(channel.getChannelInfo());
          this.service.channelParted(channel,client.getNick());
        }
 else {
          Channel channel=server.getChannel(parts[1]);
          client.partChannel(channel.getChannelInfo(),parts[2]);
          this.service.channelParted(channel,client.getNick());
        }
      }
    }
  }
 else   if ((command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) && parts.length > 1) {
    if (!client.userExists(parts[1])) {
      client.changeNick(parts[1]);
    }
 else {
      this.service.activeChannelMessageReceived(this.activity.getCurrentChannel(),Channel.createError(SpannedString.valueOf("String_Node_Str" + parts[1])));
    }
  }
 else   if ((command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) && parts.length > 1) {
    for (int i=1; i < parts.length; i++) {
      client.sendRawLine("String_Node_Str" + parts[i]);
    }
  }
 else   if (command.equalsIgnoreCase("String_Node_Str") && parts.length > 1) {
    Channel channel;
    if (this.looksLikeChannel(parts[1])) {
      channel=server.getChannel(parts[1]);
    }
 else {
      channel=this.activity.getCurrentChannel();
    }
    client.setMode(channel.getChannelInfo(),message.substring("String_Node_Str".length() + 1,message.length()));
  }
 else   if (command.equalsIgnoreCase("String_Node_Str") && parts.length > 2) {
    if (this.looksLikeChannel(parts[1])) {
      client.sendCTCPCommand(server.getChannel(parts[1]).getChannelInfo(),parts[2]);
    }
 else {
      client.sendCTCPCommand(parts[1],parts[2]);
    }
  }
 else   if ((command.equalsIgnoreCase("String_Node_Str") || command.equalsIgnoreCase("String_Node_Str")) && parts.length > 1) {
    client.sendRawLine(message.substring(parts[0].length() + 1,message.length()));
  }
}