Can somebody explain this odd behavior when working with ThreadPool?

You’re closing over the loop variable, which gives you an unexpected result. Try this instead:

foreach(string item in Items)
{
    string item2 = item;
    Console.WriteLine("Adding {0} to ThreadPool", item2);
    ThreadPool.QueueUserWorkItem
    (
        delegate
        {
            Load(item2, this.progCall, this.compCall);
        }
    );
    numThreads++;

    Thread.Sleep(100);//Remove this line

}

References

• Closing over the Loop Variable in C#
• Closing over the loop variable considered harmful