The uncommon ion effect occurs with the addition of an inert salt to a solution containing the members of the solubility equilibrium reaction to be studied. Upon the addition of an inert salt to the solution, the solubility of the original salt of the solution increases.
This is due to the fact that the ions of the inert salt surround the ions of the original salt, diverting charge that would normally be used as an attractive force for the possible left shift of the equilbrium reaction of the original salt. Thus the addition of the inert salt changes the solubility based extensively on the charge of the ions of the inert salt, the activity coefficient of the ions of the inert salt, and the size of the ions of the inert salt.
This is obviously different from the common ion effect, which is simply a result of Le Chatelier's Principle.