This week's book giveaway is in the Java in General forum. We're giving away four copies of Think Java: How to Think Like a Computer Scientist and have Allen B. Downey & Chris Mayfield on-line! See this thread for details.
You don't need multiple threads to get that exception. All you need is for the list to be structurally modified other than through the active iterator. It states this right in the docs for the exception.