Consider the following measure.
[Sales Amount] > 0 && [Total Cost] > 0,
[Sales Amount] - [Total Cost]
The basic idea is that the difference between Sales Amount and Total Cost should be evaluated only whether both measures are greater than zero. In such a condition, the DAX engine produces a query plan that evaluates each measure twice. This is visible in the storage engine requests generated for the following query.
Read on to see how Marco avoids this performance issue.