This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan 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.