What are some common mistakes students make with solubility?
Numerically - incorrect equation setup.
Conceptually - forgetting that "like dissolves like" and that "solubility" is a relative continuum.
Calculating solution values from equilibrium conditions and solubility constants requires careful attention to the product and reactant definitions in the equation, and a properly balanced chemical reaction, where pertinent.
Forgetting that solubility is "relative" to other substances leads to errors in stating that something is "soluble" or not. Knowing what orders of magnitude of concentrations will result in precipitates or not, particularly in the presence of other compounds.