Low-angle X-ray diffraction profiles can provide detailed information about the multilayer periodicity and thickness of the individual layers that make up a multilayer. The effect of discrete and continuous random cumulative fluctuations of the layer thicknesses on the diffraction profiles of single films, bilayers and multilayers is discussed. It is shown that the calculated low-angle profiles of single films can distinguish between discrete and continuous thickness errors. In bilayers and multilayers, however, no difference between these thickness errors can be seen and only an overall thickness error may be extracted.