Why array behaves different when different thread access it

I need to parse large text that is similar to XML. Because the text it is not in memory ( I have a StreamReader object) placing that stream on memory is where I take the most time. So on one thread I place that stream into an array (memory). And I have another thread that process that array. But I am having wierd behavieours. For example take a look at this image:

Note that listToProcess[counter] = buffer and right now that should be listToProcess[10] = buffer Note that the debugger says that listToProcess[10]=null why!? . the other thread just reads the items it does not modify them. At first I thought that maybe the other thread was making that item = null but that is not the case. why am I experiencing this behavior?

In case you want to see my code here it is:

        Semaphore sem = new Semaphore(0, 1000000);
        bool w;
        bool done = false;

        // this task is responsible for parsing text created by main thread. Main thread
        // reads text from the stream and places chunks in listToProces[]
        var task1 = Task.Factory.StartNew(() =>
        {
            sem.WaitOne(); // wait so there are items on list (listToProcess) to work with                
                                // counter to identify which chunk of char[] in listToProcess we are ading to the dictionary
                int indexOnList = 0;

                while (true)
                {
                    if (listToProcess[indexOnList] == null)
                    {
                        if (done)
                            break;

                        w = true;
                        sem.WaitOne();
                        w = false;



                        if (done)
                            break;

                        if (listToProcess[indexOnList] == null)
                        {
                            throw new NotFiniteNumberException();
                        }
                    }

                    // add chunk to dictionary
                    ProcessChunk(listToProcess[indexOnList]);

                    indexOnList++;
                }

        }); // close task1

        bool releaseSem = false;

        // this main thread is responsible for placing the streamreader into chunks of char[] so that
        // task1 can start processing those chunks
        int counter = 0;
        while (true)
        {
            char[] buffer = new char[2048];

            // unparsedDebugInfo is a streamReader object
            var charsRead = unparsedDebugInfo.Read(buffer, 0, buffer.Length);

            if (charsRead < 1)
            {
                listToProcess[counter] = pattern;
                break;
            }

            listToProcess[counter] = buffer;
            counter++;

            if (releaseSem)
            {
                sem.Release();
                releaseSem = false;
            }

            if (counter == 10 || w)
            {
                releaseSem = true;
            }
        }

        done = true;

        sem.Release();
       task1.Wait();

Edit

Sorry in other words why do I hit this break point:

I thought that counter was the problem but maybe I am doing something wrong with the semaphore...