>Let me introduce you to my friend, Mister ConcurrentModificationException. He's a wily fellow-- he rarely pops up when you're doing local development, but he has this nasty habit of showing up in production whenever you start hitting a decent load.

You do realize that concurrency and parallelization are very different topics right? One is a necessary property, the other is one of oppurtunity. It makes sense that collections designed for concurrency would suck at parallelism.