Confounding
The design of an experiment may make it impossible to disentangle the effects of the treatment and other characteristics of the experimental uits. If the treatment is perfectly correlated with another variable, the effects of the two variables cannot be distinguished. The treatment and variable are then said to be confounded.
It is particularly important to avoid confounding in an experiment.
In an experiment, treatment A was applied to 10 experimental units in 2010 and treatment B was used on 10 similar units in 2011.
It is impossible to tell whether the higher mean response for treatment A than treatment B was caused by the different treatments or other differences between the two years.