When making cold process soap sometimes the unexpected can happen. One such occurrence is the cracking of a log of soap while it is still in the mould after recently being poured.

This has happened because your soap has begun to set on the outside, but the inside has heated up and expanded to the point it has cracked the shell that was forming on the outside.  The centre will be going through gel (which most people find desirable).

So, What do we do to Fix It?  
In future batches you may wish to soap at a lower temperature.  This will reduces the overall temperature and it should not get to a point where the heated centre cracks the top of the soap.  If you are using milks or sugar containing products such as honey this is definitely a good option as the raised temperature will also "cook" the sugars in these sorts of ingredients and will cause them to darken your end product.

Another option is to insulate the soap so the top also heats up at the same rate and the whole log will go through gel and the cracking will not occur. (Not recommended for the sugar containing recipes listed above as they will darken)

Is Cracking a Problem?
While the cracking through the loaf may initially be disturbing it is most likely on the top few millimetres of the crust and is only aesthetic.  As the log cools a lot of the crack should close back up again with the contraction of the contents of the log returning to its starting point.  The small fracture that may still be visible can easily be corrected by rubbing a gloved finger over the surface to gently refill the crevice with the still soft soap.  In the end it is likely only you will ever know there was a moment of doubt.  You can rest assured there is nothing technically wrong with your soap and it is absolutely fine to use when it has completed its cure time.