An idle pulley came apart like that on a car at a Dragon meet maybe two years ago. The car was making noises, and one of the guys diagnosed it as a bad idler.
They found a new one at a parts store in (I think it was) Murphy, NC; when they took the bolt out of the old one, it came apart in two pieces.

I dont know if all of them were made like that, or only certain years, or only certain aftermarket ones were - cause the one I changed on my Graphite roadster did not come apart like that.