The cylinder only fits in the circular hole.

You can't assume that you always have a cylinder or a cube to fit into the mold. Heck, you may end up with an icosidigon and have only a triangle to try to shove it into.

As amazing as the Y Combinator idea may sound to some, you also have to have a healthy sense of skepticism and apply it appropriately to every situation and oppourtunity that comes your way. Make it work for you, not you for it.