i talked with and showed a coworker who is a structural engineer here both pictures of a new spacer and the broken posted by the OP. here are his quick findings::
1. its easy to shout out material defect since these are light weight aluminum alloy mixed materials. just a tad bit of iron oxide in the mix and failures on such a high toleranced part is bound to happen.
2. and this one is the most interesting one to me. there is a rather large relief cut right before the thin ring. that lets the rim sit squarely on the spacer. he mentioned if you were to get a small scratch or nick along that edge right around the relief area, it wouldnt take long for a small crack to form and for it to migrate completley around the hub as the wheel spun. causing a clean quick "snapping off" look that he saw in the pic posted. he said its very important that the wheel face and the spacer be free of any type of debree, and that it wouldnt take anything large to start that scatch that led to the failure. dirt/rock, metal shaving from the shops floor, ect....
too many variables to throw in but thats my version of stirring up this mess...
