If primordial fluctuations follow Gaussian distribution, higher-order cumulants of cosmic fields could reflect nonlinear mode coupling, and would provide useful information about the gravitational instability picture of structure formation. We show that, in the usual case that the observed sample has a limited volume, their expected deviation (sample variance) from their universal values is nonvanishing, even in linear theory. As a result, we find that the relative sample variance of the skewness of the velocity divergence field can be as large as similar to 30% in current velocity surveys.