I haver broken both Cinelli 1R and XA stems and posted to this effect several times previously. The last time it happened I was very fortunate that I was not 100 metres further into the ride. It broke about 25m before I crested an overpass. I was going slowly and there was no trafiic, so when the bars fell away, I was simply able to disengage my right foot and step off. If I had been going down the other side at 40-50kph, the consequences would have been very serious. I was also fortunate to be using Look clipless pedals. If I had been using toe clips and straps. I would surely have kissed asphalt.

As stated by Jim, they break across the center of the relief for the insert. This relief for reduces the metal in this area by about 50% and the problem is compounded by theserrations on the inside of the clamp which act as stress risers.

All my active bicycles have had their XA and 1R stems replaced with 1A.