Since its invention in 1986, the Atomic Force Microscope (AFM) is intensively being used in all branches of science for very high resolution mapping of the topography of surfaces, at sub-nanometer vertical resolution. The usual quantitative analysis of surface roughness is to compile the various moments of the height distribution of an image (e.g., mean height, standard deviation of surface heights, skewness, etc). Perhaps the parameter mostly used for characterizing the surface roughness is the RMS roughness. In this work, some of the limitations of this parameters are shown. Theoretical and very different surfaces with different spatial variations are constructed showing the same RMS roughness.