The phrase "small crack" refers to a tiny break or opening in something, like a surface or an object. It can also be used figuratively to suggest a minor weakness or vulnerability in someone's character or in a situation. Full definition
As babies grow, the diapers are subjected to stretching, which can result in small cracks on the diaper, often nearest the front closing tabs. (thinking-about-cloth-diapers.com)
This dangerous gas enters your home through small cracks and holes in the foundation. (amerispec.com)
Even small cracks and bumps in the road produce a hard, sharp response. (cars.com)