Program: 177361
Project: 177361
Buggy source:
@Override @LiveRead public SpoutChunk getChunk(int x,int y,int z,LoadOption loadopt){
  x&=CHUNKS.MASK;
  y&=CHUNKS.MASK;
  z&=CHUNKS.MASK;
  final SpoutChunk chunk=chunks[x][y][z].get();
  if (chunk != null) {
    if (loadopt.loadIfNeeded()) {
      if (!chunk.cancelUnload()) {
        throw new IllegalStateException("String_Node_Str");
      }
    }
    return chunk;
  }
  SpoutChunk newChunk=null;
  boolean generated=false;
  final AtomicReference<SpoutChunk> chunkReference=chunks[x][y][z];
  ChunkDataForRegion dataForRegion=null;
  if (loadopt.loadIfNeeded() && this.inputStreamExists(x,y,z)) {
    dataForRegion=new ChunkDataForRegion();
    newChunk=WorldFiles.loadChunk(this,x,y,z,this.getChunkInputStream(x,y,z),dataForRegion);
  }
  if (loadopt.generateIfNeeded() && newChunk == null) {
    newChunk=generateChunk(x,y,z);
    generated=true;
  }
  if (newChunk == null) {
    return null;
  }
  while (true) {
    if (chunkReference.compareAndSet(null,newChunk)) {
      newChunk.notifyColumn();
      numberActiveChunks.incrementAndGet();
      if (dataForRegion != null) {
        for (        SpoutEntity entity : dataForRegion.loadedEntities) {
          entity.setupInitialChunk(entity.getTransform().getTransform());
          addEntity(entity);
        }
        dynamicBlockTree.addDynamicBlockUpdates(dataForRegion.loadedUpdates);
      }
      occupiedChunksQueue.add(newChunk);
      Spout.getEventManager().callDelayedEvent(new ChunkLoadEvent(newChunk,generated));
      return newChunk;
    }
    newChunk.setUnloadedUnchecked();
    SpoutChunk oldChunk=chunkReference.get();
    if (oldChunk != null) {
      if (loadopt.loadIfNeeded()) {
        if (!oldChunk.cancelUnload()) {
          throw new IllegalStateException("String_Node_Str");
        }
      }
      return oldChunk;
    }
  }
}

Fixed version (ground truth):
@Override @LiveRead public SpoutChunk getChunk(int x,int y,int z,LoadOption loadopt){
  x&=CHUNKS.MASK;
  y&=CHUNKS.MASK;
  z&=CHUNKS.MASK;
  final SpoutChunk chunk=chunks[x][y][z].get();
  if (chunk != null) {
    if (loadopt.loadIfNeeded()) {
      if (!chunk.cancelUnload()) {
        throw new IllegalStateException("String_Node_Str");
      }
    }
    return chunk;
  }
  SpoutChunk newChunk=null;
  boolean generated=false;
  final AtomicReference<SpoutChunk> chunkReference=chunks[x][y][z];
  ChunkDataForRegion dataForRegion=null;
  if (loadopt.loadIfNeeded() && this.inputStreamExists(x,y,z)) {
    dataForRegion=new ChunkDataForRegion();
    newChunk=WorldFiles.loadChunk(this,x,y,z,this.getChunkInputStream(x,y,z),dataForRegion);
  }
  if (loadopt.generateIfNeeded() && newChunk == null) {
    newChunk=generateChunk(x,y,z);
    generated=true;
  }
  if (newChunk == null) {
    return null;
  }
  while (true) {
    if (chunkReference.compareAndSet(null,newChunk)) {
      newChunk.notifyColumn();
      numberActiveChunks.incrementAndGet();
      if (dataForRegion != null) {
        for (        SpoutEntity entity : dataForRegion.loadedEntities) {
          entity.setupInitialChunk(entity.getTransform().getTransform());
          entityManager.addEntity(entity);
        }
        dynamicBlockTree.addDynamicBlockUpdates(dataForRegion.loadedUpdates);
      }
      occupiedChunksQueue.add(newChunk);
      Spout.getEventManager().callDelayedEvent(new ChunkLoadEvent(newChunk,generated));
      return newChunk;
    }
    newChunk.setUnloadedUnchecked();
    SpoutChunk oldChunk=chunkReference.get();
    if (oldChunk != null) {
      if (loadopt.loadIfNeeded()) {
        if (!oldChunk.cancelUnload()) {
          throw new IllegalStateException("String_Node_Str");
        }
      }
      return oldChunk;
    }
  }
}