soxinbox wrote:

What can cause this kind of
crack? Is this indicative of a manufacturing problem?

In addition to dropping the plug (as mentioned by others), this can be caused by
using a standard deep-well socket instead of a plug socket. Tilting the socket
when trying to remove it after tightening the plug is another way to break one.
Hitting the plug accidentally while working on something else is another.
Properly trained mechanics will not do either of the first two; amateurs might.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.