Mutations in cells can lead to three things: being changed for the worse, changed for the better, or being changed in a way that neither negatively or positively effects the well being of the cell. In the case of the shoe factory, mistakes in copying the shoe design (much like mistakes in copying the DNA within cells) would lead to one of the three outcomes listed above. The outcome may be an improvement in the shoes produced, but it could also make the shoes worse.
Work Step by Step
During reproduction, a cell copies its DNA in order to pass a copy on to each of its daughter cells. However there can be mistakes, called mutations, in the copy of DNA. The mutation has the potential to increase the survival chances of the cell, giving the mutated cell an edge on the competition in the process of natural selection, worsen the cells chances of survival, or have no effect on the survival capabilities of the cell. The shoes in the factory are like the DNA within the cell - mistakes could lead to improvements, weaknesses, or nothing at all.