Yes, they break. From what I've seen, it is usually the disk or power supply sub-system(s).
It would be a pretty rare thing to run across, but trust me on this: as funky as the AS-400 is, you should have seen the System 34/36/38's.
Gawd, it was like hitting yourself in the forehead with a hammer. All you could think about was "how good it will feel when I stop..."