Electrolysis of concentrated sulphuric acid
This is because in concentrated sulfuric acid solution equilibrium condition will favour more water molecules than oxidation of water into protons. So there will be an over potential required (to go against the equilibrium) , that is extra potential beyond the theoretical reduction potential derived from thermodynamics to complete the reaction. So the total potential required will be theoretical Plus overpotential. In case of oxidation of sulphate reduction potential will be much less that for water ,thus oxidation of sulphate happens.