Program: 11807
Project: 11807
Buggy source:
public PacketPlayOutChunkData asPacket(){
  try {
    return executor.submitTask(new Callable<PacketPlayOutChunkData>(){
      @Override public PacketPlayOutChunkData call() throws Exception {
        PacketPlayOutChunkData packet=new PacketPlayOutChunkData();
        int bitmask=(1 << sections.length) - 1;
        int count=sections.length;
        int size=0;
        int sectionSize=ChunkSection.LENGTH * 5 / 2;
        if (world.dimension() == Dimension.OVERWORLD)         sectionSize+=ChunkSection.LENGTH / 2;
        size+=count * sectionSize + 256;
        for (        ChunkSection section : sections) {
          if (section == null)           continue;
          for (          char c : section.types()) {
            data.write(c & 0xff);
            data.write(c >> 8);
          }
        }
        for (        ChunkSection section : sections) {
          try {
            data.write(section.blockLight);
          }
 catch (          IOException e) {
            TridentLogger.error(e);
          }
        }
        for (        ChunkSection section : sections) {
          try {
            data.write(section.skyLight);
          }
 catch (          IOException e) {
            TridentLogger.error(e);
          }
        }
        for (int i=0; i < 256; i+=1) {
          data.write(0);
        }
        packet.set("String_Node_Str",location);
        packet.set("String_Node_Str",(short)bitmask);
        packet.set("String_Node_Str",data.toByteArray());
        data.reset();
        return packet;
      }
    }
).get();
  }
 catch (  InterruptedException|ExecutionException e) {
    TridentLogger.error(e);
    return null;
  }
}

Fixed version (ground truth):
public PacketPlayOutChunkData asPacket(){
  try {
    return executor.submitTask(new Callable<PacketPlayOutChunkData>(){
      @Override public PacketPlayOutChunkData call() throws Exception {
        PacketPlayOutChunkData packet=new PacketPlayOutChunkData();
        int bitmask=(1 << sections.length) - 1;
        ByteArrayOutputStream data=new ByteArrayOutputStream();
        for (        ChunkSection section : sections) {
          if (section == null)           continue;
          for (          char c : section.types()) {
            data.write(c & 0xff);
            data.write(c >> 8);
          }
        }
        for (        ChunkSection section : sections) {
          try {
            data.write(section.blockLight);
          }
 catch (          IOException e) {
            TridentLogger.error(e);
          }
        }
        for (        ChunkSection section : sections) {
          try {
            data.write(section.skyLight);
          }
 catch (          IOException e) {
            TridentLogger.error(e);
          }
        }
        for (int i=0; i < 256; i+=1) {
          data.write(0);
        }
        packet.set("String_Node_Str",location);
        packet.set("String_Node_Str",(short)bitmask);
        packet.set("String_Node_Str",data.toByteArray());
        data.reset();
        return packet;
      }
    }
).get();
  }
 catch (  InterruptedException|ExecutionException e) {
    TridentLogger.error(e);
    return null;
  }
}