If the temperature in the fireplace is too low when the fireplace is turned on, cracking is possible. Fortunately our gas logs are reinforced with steel rods to keep the same functionality and keep everything together, but other products, not so. Cracking is not too common, but it the chance of it happening is there.
Unfortunately rapid changes in temperature will cause these due to the nature of physics and until advances in material science can give us cement that does not crack under rapid temperature change, our users should be advised to be cautious when using our products.
To help try to avoid it, make sure not to go from cold to the highest setting immediately and also try to turn your fireplace on earlier in the night rather than later to minimize the temperature difference.
Unfortunately cracking is not a warrantable cause, but cracked items may be replaced as a customer service. If cracking occurs in a fireball set or a set with many of the same pieces, it may be easiest to move the cracked pieces to the back or the sides where they are not seen.