My first thought was that this might be due to someone prying the stem open a bit too far when installing a new bar.

The alternate, where someone might initiate a small crack by using a bar that has a slightly small O.D., seems like a good candidate too.

I've got two bikes with 3TTT bars, but also have 3TTT stems. One of these sets was purchased new by me. The other is on a bike where it seems to be original equipment. Probably hasn't been man-handled in the past.

Has anyone closely inspected the broken stems? Is there a dark area of the crack and a light area, indicating that the crack took a long time to propagate? If not, there's likely no chance of catching it by visual inspection.

Steve