Fluctuations in plasma potential bring about an apparent broadening in the ion energy distribution when it is derived from theI‐Vcharacteristic of a retarding grid energy analyzer. This effect may have detrimental consequences for the prospects of applying energy analyzers to measure diffusion of the background distribution when electrostatic turbulence is present. The effect is demonstrated by inducing spatially homogeneous oscillations of the plasma potential in a beam‐plasma system, applying a directional ion energy analyzer for beam detection. A theoretical analysis is presented, and quantitative agreement with the experimental results is obtained.