Originally Posted by
redlude97
Aluminum doesn't fracture in layers like this. See the red circles. The break should be more clean if it was overtightening. The green circles show signs of corrosion on the unbroken areas of the stem.
It can if it was only cracked at the top and the final part failed with a downward force (rider). The hard anno on the surface of the stem can make the last section peel like that.
There does look like corrosion as well. There could be more than one factor involved in the failure.