One investigates the error of the projection methods in solving the problem regarding the shortest distance from a point to a closed convex set M in a Hilbert space H. One introduces the concept of equipotential convexity of a set M and one gives an estimate of the error of the mentioned methods in terms of the best approximation by elements of the intersection X ∩ M, where X is the projection plane in H. One gives an example which shows that, in general, without the condition of equipotential convexity such an estimate cannot be obtained.