How to remove items from a Slicer that are not actually in the data set that the Slicer filters.
This becomes important once you start deleting values from your data source for a Table or Pivot Table.
I really don't know why Excel makes us go through the step of changing settings in order to make this work, and I don't know why this is default behaviour for Slicers and Pivot Tables, but, at least its an easy problem to solve.
For more tips on slicers, check out this tutorial: 3 Slicer Tricks for Pivot Tables in Excel