Here are two pictures I took of a Bayis shel Rosh before it has been completed. I hope it helps illustrate a bit more clearly what the 'cracks' are in the pictures that R. Moshe Weiner posted. As I hope you can see, they are just the folds of the leather to make the four batim of the shel rosh. Sometimes when there's a bit too much leather for each compartment, the extra gets squeezed in the middle making these folds that look like crack along the whole length of the bayis. It's of course not a problem of any kind.