Fast-staining protocols based on the use of Coomassie blue dye for SDS-PAGE separated proteins, represent a quick and simple solution for protein visualization. It has been shown however, that in some cases a phenomenon of missing spots or spot discoloration may be observed in the proteome pattern when the standard fast-staining protocol is used. In this work, it is demonstrated that this occurrence is affected by the biological variability of samples, and therefore, cannot be observed in all samples. Moreover, it is demonstrated that the phenomenon is manifested exclusively in nonfixed gels, and that including a fixation step into the fast-staining protocol prevented this phenomenon. In conclusion, it has been demonstrated that standard Coomassie blue dye based fast staining for SDS-PAGE resolved proteins is affected by the biological variability of samples in nonfixed gels.