There are no perfect instruments in the real world. Every scientist knows this, but as long as s/he can manage the observational error, research can continue.
Everytime we repeat a measurement with a sensitive instrument, we obtain slightly different results. The common StatisticalModel? we use is that the error has two additive parts:
The SystematicError? is sometimes called StatisticalBias?. We control it by using very carefully standardized procedures. Part of the education in every ScienCe is how to use the standard instruments of the discipline.
The RandomError? (or RandomVariation?) is due to factors which we cannot (or do not) control. It may be too expensive or we may be too ignorant of these factors to control them each time we measure. It may even be that whatever we are trying to measure is changing (see DynamicModels?).