In the case of the “nail problem,”

In the case of the “nail problem,” the nail into the wall is the whole
system and the rod of the nail is our element with conflicting physical requirements. Its characteristic “shape of cross-section” or
“profile” is exactly the feature which should satisfy the conflicting requirements. Since this profile should be circular and should be noncircular. Thus we have almost formulated a physical contradiction: “Nail’s rod profile should be noncircular to eliminate
rotation of nail in the wall and should be circular for preservation of the original nail manufacturing.”