The "contamination" comes from the lab, it is already in the vial. If you were to do some research, you would find that every cell is not genetically identical. And some of this is actually on purpose. Some of the genetic variations you would find allow the yeast to quickly go through its respiration cycle, reach its proper attenuation... etc. And yes, there is bad contamination in the vial too. Will you notice? probably not. But when the yeast vial is past its "best by" date, the yeast cells you want may not be as populous as the strains you dont want much of, due to the differing hardy nature of living organisms. This discussion is obviously way deeper than originally intended, but interesting none the less. This is also why a brewer only repitches his yeast several times rather than indefinately.