Failed plug

I just had some engine instruments installed, and I was going to return my
plane to service. I did two power runs, aborting takeoff right before
rotation. These were normal and uneventful. I popped the cowling and
inspected the engine and all was well. I then taxied out for a test flight.
During the run up the mag check failed, and was severe enough that I did not
suspect carbon fowling. I taxied back and had my mechanic help me check the
plugs. One plug was bad and we replaced it. The engine worked fine and the
flight went on without incident.

Later I took a close look at the failed plug. It has a crack from about 1/4
inch from the seat to 1/8 inch from the threads for the plug wire retainer.
The crack is not opened up, but is a distinct fine crack about 2 inches
long. Is this a normal failure for a spark plug? What can cause this kind of
crack? Is this indicative of a manufacturing problem?