Ranging from track interpolation techniques through model atmospheres to the stochastic nature of the IMF, there are many uncertainties which need to be taken into account when modelling HR diagrams or performing population synthesis, particularly if comparison with actual data is sought. In this paper, we highlight (1) the problem of discontinuities along evolutionary tracks of massive stars (M> 8 Mo), showing that inconsistencies appear in the computation of the corresponding isochrones, and (2) the sampling fluctuations produced by the stochastic nature of the IMF, presenting a statistical formalism to estimate the dispersion in any given observable of a stellar population due to sampling effects which bypasses the need of performing Monte Carlo simulations.