James,

I have doubts that the reason was, the X mold cracked.
Certain runs may have cracked, but probably certain runs of a lot of molds cracked. I think it probably had to do with the formula of the clay or how long the run was baked.
example:
2 runs mixed and ran at the same time, different molds. 1 baked for 10 minutes and 1 baked for 11 minutes. Just an opinion!!!