We had this identical failure in one of our SuperShifters during our first unbagging window this season. Discovering the issue at that time literally saved our season (otherwise, we likely would have failed the shaft during our first event).
We were cantilevering the load on the shaft (chain run in either direction, but not exactly horizontal) and were surprised when the aluminum shaft failed. I know that the snap ring groove is a stress concentrator, but was still surprised by the failure. We changed to steel and have not had problems since (this is not an indictment of aluminum shafts, but we wanted to go super-conservative after one failure).
Here is a close-up of our drive train geometry for reference:
http://www.flickr.com/photos/team341...57628843002609