It is possible for a suspected confounder Z to be a mediator (or on the causal path) between X and Y. Therefore, controlling for Z in such a situation would lead to major misinterpretations as controlling for Z could effectively block the effect of X in the causal diagram. 

University of Michigan - Ann Arbor

From the text:

A confounder of X and Y is a variable Z that:
1. is associated with X in the population at large
2. associated with Y among the people who have not been exposed to the treatment in X.
3. (Recently) Z should not be on the causal path between X and Y.

Classical Epidemiological Definition of a Confounder

Pearl, J., & Mackenzie, D. (2018). $\mathit{The \ Book \ of \ Why: \ The \ New \ Science \ of \ Cause \ and \ Effect.}$ Basic Books.

Learn Before

Related